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BIOLOGY 


INTERNATIONAL    CHEMICAL    SERIES 
H.  P.  TALBOT,  PH.  D.,  CONSULTING  EDITOR 


APPLIED 
COLLOID   CHEMISTRY 


GENERAL  THEORY 


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APPLIED 
COLLOID    CHEMISTRY 

GENERAL    THEORY 


BY 


WILDER  D.  BANCROFT 
//  '/ 

PROFESSOR   OF   HHYSICAL   CHEMISTRY   AT1  CORNELL,  UNIVERSITY 


FIRST  EDITION 


McGRAW-HILL  BOOK  COMPANY,  INC. 

NEW  YORK:    370  SEVENTH  AVENUE 

LONDON:    648  BOUVERIE  ST.,  E.  C.  4 

1921 


COPYRIGHT,  1921,  BY  THE 
McGRAw-HiLL  BOOK  COMPANY,  INC, 


MAPLK     PRKSS     T  O  K  K    PA 


PREFACE 

The  earlier  books  on  colloid  chemistry  presented  the  subject 
empirically  because  no  other  method  was  then  possible.  While 
we  do  not  now  know  much  about  gelatinous  precipitates  and 
jellies,  the  theory  of  the  rest  of  the  subject  is  in  fairly  good  shape 
and  consequently  I  have  written  this  book  deductively.  While 
my  point  of  view  differs  considerably  from  Freundlich's,  it  is  a 
pleasure  to  acknowledge  my  debt  to  him  for  what  he  did  in  his 
admirable  book. 

This  volume  on  general  theory  should  be  followed  by  at  least 
one  volume  on  each  of  the  following  subjects:  silicate  industries; 
paints  and  varnishes;  plastics;  fibers  and  dyeing;  photochemistry 
and  photography;  petroleum  industries;  ore  flotation  and  allied 
subjects;  foods  and  beverages;  soils  and  crops;  biology  and  medi- 
cine. Allowing  a  minimum  of  three  years  to  a  volume,  it  seems 
very  improbable  that  I  shall  write  all  of  these  single-handed. 
With  the  general  theory  of  the  subject  once  cleared  up,  there  is  no 
reason  why  the  other  volumes  should  not  be  written  by  people 
whose  knowledge  of  the  special  subjects  is  much  greater  than 
my  own. 

I  am  indebted  to  Messrs.  Bray;  Briggs,  Clowes,  Holmes, 
Johnston  and  Lamb  for  reading  the  manuscript  and  for  critical 
comments  on  it. 

CORNELL  UNIVERSITY, 
November  1,  1920. 


432049 


TABLE  OF  CONTENTS 

PAGE 

INTRODUCTION 1 

CHAPTER  I 

ADSORPTION  OF  GAS  OR  VAPOR  BY  SOLID ........  3 

Adsorption  of  Gas  or  Vapor  by  Solid 3 

The  Adsorption  Isotherm 29 

Chemical  Reactions 40 

CHAPTER  II 

ADSORPTION  OF  VAPOR  BY  LIQUID,  AND  OF  LIQUID  AND  SOLID  BY  SOLID 

AND  LIQUID 65 

Adsorption  of  Vapor  by  Liquid 65 

Adsorption  of  Liquid  by  Solid 67 

Adsorption  of  Solid  by  Solid v 78 

Adsorption  of  Liquid  by  Liquid 81 

Adsorption  of  Solid  by  Liquid 83 

CHAPTER  III 

ADSORPTION  FROM  SOLUTION 90 

Adsorption  from  Solution  by  Solid 90 

The  Adsorption  Isotherm 100 

Abnormal  Adsorption 106 

Negative  Adsorption 108 

Reversibility  of  Equilibrium 110 

Specificity  of  Adsorption Ill 

Adsorption  of  Several  Solutes 113 

Chemical  Action .    .    .    ,    .    .    ...,.    .    .    .  120 

Adsorption  from  Solution  by  Liquid 126 

CHAPTER  IV 

SURFACE  TENSION — BROWNIAN  MOVEMENTS 129 

Adsorption  and  Surface  Tension 129 

Brownian  Movements    .    .    .    ,  • .    .    ...    .    .  • .    .    .    .    .  V  .    .  136 

CHAPTER  V 

COALESCENCE •'.    ............  146 

Coalescence  of  Liquids 146 

Coalescence  of  Solids 147 

Types  of  Precipitates      .    .    ...    .  '.    .    .    . 161 

Theory  of  Peptization     ....  '.'.•.../.*•'. 166 

vii 


viii  CONTENTS 

CHAPTER  VI 

PAGE 

PREPARATION  OF  COLLOIDAL  SOLUTIONS 172 

Condensation  Methods 172 

Dispersion  Methods 177 

CHAPTER  VII 

PROPERTIES  OF  COLLOIDAL  SOLUTIONS 182 

General  Properties 182 

Viscosity 190 

Color.   " 196 

Electrical  Properties 206 

Stability  of  Colloidal  Solutions 212 

CHAPTER  VIII 

JELLIES  AND  GELATINOUS  PRECIPITATES 236 

Structure 236 

Vapor  Pressure  Relations 245 

Diffusion 258 

CHAPTER  IX 

EMULSIONS  AND  FOAMS 260 

Emulsions .    .    . 260 

Foams 268 

CHAPTER  X 

NON- AQUEOUS  COLLOIDAL  SOLUTIONS 274 

CHAPTER  XI 

FOG  AND  SMOKE 276 

Fog v 276 

Smoke .  286 

Precipitation  of  Solid      ......    .  ' 300 

CHAPTER  XII 

GASES  AND  SOLIDS  IN  SOLIDS     ..,,..• 304 

Gases  in  Solids.    .    .    .    .    .    .    .    .  ,.    ....    .  r .  304 

Solids  in  Solids .  \\  ..» " 305 

CHAPTER  XIII 

THICKNESS  OF  SURFACE  FILMS      '. 314 

AUTHOR  INDEX 321 

SUBJECT  INDEX  .  333 


APPLIED  COLLOID  CHEMISTRY 


INTRODUCTION 

In  1861  Graham1  pointed  out  that  substances  like  potassium 
hydroxide,  potassium  sulphate,  magnesium  sulphate,  sugar, 
and  alcohol  diffuse  much  more  rapidly  in  water  than  hydrous 
silicic  acid,  hydrous  alumina,  starch,  dextrin,  the  gums,  albumin, 
tannin,  gelatine,  etc.  Since  these  latter  substances  occur  usually 
in  a  non-crystalline  or  even  gelatinous  form,  Graham  suggested 
calling  them  colloids  from  the  French  word  for  glue.  Substances 
which  diffused  rapidly  were  to  be  called  crystalloids  because  many 
of  them  crystallized  readily.  Graham  believed  that  the  distinction 
between  a  crystalloid  and  a  colloid  was  fundamental  and  was  due 
to  some  molecular  condition.  Though  modern  colloid  chemistry 
begins  with  Graham,2  his  distinction  between  crystalloids  and 
colloids  has  been  dropped.  A  colloidal  substance  is  not  neces- 
sarily amorphous,  for  colloidal  gold  is  certainly  crystalline  at 
times  and  possibly  always.  We  now  speak  of  a  colloidal  state 
instead  of  a  colloidal  substance,  and  we  call  any  phase  colloidal 
when  it  is  sufficiently  finely  divided  or  dispersed,3  without  com- 
mitting ourselves  definitely  as  to  what  degree  of  subdivision  is 
necessary  in  any  particular  case.  The  subdivided  phase  is 
often  called  the  internal  or  dispersed  phase  and  the  enveloping 
phase  the  external  phase  or  the  dispersing  phase.  In  fog  the 
rain  drops  are  the  dispersed  phase  and  the  air  the  dispersing 
phase,  while  the  solid  particles  are  the  internal  phase  in  the  case 
of  smoke.  When  a  milky  liquid  is  obtained  by  precipitating 
barium  sulphate  in  the  cold,  the  barium  sulphate  is  the  dispersed 

1  Phil.  Trans.  151,  183  (1861);  Jour.  Chem.  Soc.,  16,  216  (1862);  17,  318 
(1864). 

2  Interesting  work  was  done  by  Selmi  in  1844;  but  it  had  no  appreciable 
effect  on  the  development  of  scientific  thought. 

3  Cf.  Wolfgang  Ostwald:  Grundriss  der  Kolloidchemie,  79  (1909). 

1 


CHEMISTRY 

phase  aot-Mfe'  Solution  the  dispersing  phase.  In  emulsions  of  oil 
in  water,  the  oil  is  the  internal  phase  because  it  is  present  in 
drops.  When  benzene  is  emulsified  to  a  solid  jelly  in  a  small 
amount  of  soap  and  water,  we  have  a  mobile  liquid  as  dispersed 
phase  in  a  viscous  mass.  Bread  or  Ivory  soap  might  be  con- 
sidered as  a  system  with  air  as  internal  phase,  and  a  fairly  solid 
external  phase.  In  a  porous  plate  or  a  fine  sponge  the  two  phases 
are,  or  may  be,  continuous  and  it  is  therefore  better  to  classify 
these  as  interlacing  systems.  True  ruby  glass  consists  of  gold 
as  dispersed  phase  in  glass  as  a  non-crystalline  dispersing  phase. 
With  salt  colored  by  exposure  to  cathode  rays,  we  have  metallic 
sodium  as  internal  phase  in  an  external  phase  of  crystalline 
sodium  chloride. 

Adopting  the  very  flexible  definition  that  a  phase  is  called  col- 
loidal when  it  is  sufficiently  finely  divided,  colloid  chemistry  is 
the  chemistry  of  bubbles,  drops,  grains,  filaments,  and  films, 
because  in  each  of  these  cases  at  least  one  dimension  of  the  phase 
is  very  small.  This  is  not  a  truly  scientific  classification  because 
a  bubble  has  a  film  round  it  and  a  film  may  be  considered  as 
made  up  of  coalescing  drops  or  grains.  It  is  a  convenient  classi- 
fication, however  for  many  purposes.  At  first  sight  colloid 
chemistry  may  not  seem  to  be  an  important  branch  of  chemistry, 
either  theoretically  or  technically;  but  this  opinion  changes  when 
we  consider  that  a  knowledge  of  colloid  chemistry  is  essential  to 
anybody  who  really  wishes  to  understand  about :  cement,  bricks, 
pottery,  porcelain,  glass,  enamels;  oils,  greases,  soaps,  candles; 
glue,  starch,  and  adhesives;  paints,  varnishes,  lacquers;  rubber, 
celluloid,  and  other  plastics;  leather,  paper,  textiles;  filaments, 
casts,  pencils,  and  crayons;  inks;  roads,  foundry  cores,  coke, 
asphalt;  graphites,  patines;  zinc,  phosphorus,  sodium,  and  alumi- 
num; contact  sulphuric  acid,  hardened  oils,  etc.;  beer,  ale,  and 
wine;  cream,  butter,  cheese,  and  caseine  products;  cooking, 
washing,  dyeing,  printing;  ore  flotation,  water  purification,  sew- 
age disposal;  smoke  prevention;  photography;  wireless  telegraphy; 
illuminants;  comets;  pharmacy;  physiology.  In  other  words, 
colloid  chemistry  is  the  chemistry  of  e very-day  life. 


CHAPTER  I 
ADSORPTION  OF  GAS  OR  VAPOR  BY  SOLID 

ADSORPTION  OF  GAS  OR  VAPOR  BY  SOLID 

In  an  extremely  subdivided  phase,  the  surface  is  very  large 
relatively  to  the  mass  of  the  phase  and  it  is  therefore  advisable 
to  discuss  the  properties  of  surfaces,  beginning  with  the  behavior 
of  solids  in  contact  with  gases  or  vapors.  All  solids  tend  to 
adsorb,  or  condense  upon  their  surface,  any  gases  or  vapors  with 
which  they  are  in  contact.  The  amount  of  adsorption  for  given 
conditions  of  pressure  and  temperature  varies  with  the  nature 
and  physical  state  of  the  solid  and  with  the  nature  of  the  gas;  in 
other  words,  the  adsorption  is  specific  or  selective.  With  the 
same  'solid  and  the  same  gas  the  amount  of  adsorption  is  greater 
the  higher  the  pressure  of  the  gas  and  the  lower  the  temperature. 
Hunter1  showed  that  many  different  gases  and  vapors  are  ad- 
sorbed by  charcoal  to  a  marked  extent.  While  his  experiments 
are  probably  not  very  accurate,  and  are  not  made  at  strictly 
comparable  temperatures  and  pressures,  they  cover  more  ground 
than  those  of  anybody  else.  Some  of  his  data  are  given  in 
Table  I.  In  Table  II  are  some  data  by  Dewar2  with  cocoanut 
charcoal  at  low  temperatures.  The  volumes  of  gas  adsorbed 
have  been  calculated  back  to  standard  conditions ;  in  other  words 
to  the  volumes  which  the  adsorbed  gases  would  occupy  at  0°  and 
760  mm  pressure.  Thus  at  — 185°  and  760  mm  the  actual  vol- 
umes of  gas  adsorbed  would  be  only  88/273,  or  approximately 
one-third,  of  the  values  given  in  Table  II.  This  correction  was 
not  applied  by  Hunter  in  his  work. 

iPhil.  Mag.  (4)  26,  364  (1863);  Jour.  Chem.  Soc.,  18,  285  (1865);  20, 
160,  (1867);  21,  186  (1868);  23,  73  (1870);  24,  76  (1871);  25,  649  (1872). 
2  Proc.  Roy.  Soc.,  74,  124  (1904). 

3 


APPLIED  COLLOID  CHEMISTRY 


TABLE  I. — ADSORPTION  OF  GASES  AND  VAPORS  BY  COCOANUT  CHARCOAL 

Volumes  gas  adsorbed  per  volume  charcoal. 
Temperature  in  Celsius  degrees. 
Pressures  in  millimeters  of  mercury. 


Gas 

Vol. 

Temp. 

Press. 

Gas 

Vol. 

Temp. 

Press. 

Acetic  acid  
Water  
Water 

83 
24 
44 

159 
159 
128 

676 
24 
624 

Acetaldehyde  
Acetaldehyde  
Ammonia 

67 
139 
22 

155 
100 
127 

687 
681 
655 

Benzene  .  . 

59 

130 

646 

Ammonia  . 

83 

70 

760 

Ethyl  alcohol  
Ethyl  alcohol.  .  

84 
111 

160 

127 

663 
644 

Ammonia  
Ammonia  

88 
96 

60 
50 

760 
760 

Ethyl  alcohol  

141 

100 

652 

Ammonia  

114 

40 

760 

Ethyl  acetate  
Carbon  tetrachloride.  . 
Carbon  tetrachloride.  . 
Methyl  alcohol  
Methyl  alcohol  
Methyl  alcohol  

72 
4 
8 
61 
127 
151 
21 

154 
155 
100 
159 
129 
100 
159 

659 
695 
636 
684 
663 
652 
660 

Ammonia  
Ammonia  
Cyanogen  
Cyanogen  
Cyanogen  
Cyanogen  
Cyanogen 

149 
176 
87 
92 
96 
99 
107 

20 
0 
70 
60 
50 
40 
20 

760 
760 
760 
760 
760 
760 
760 

Chloroform  
Acetone  ...  . 

30 

68 

100 
157 

651 
671 

Cyanogen  
Hydrochloric  acid 

114 
60 

0 
100 

760 
668 

Acetone  

105 

100 

642 

Nitrous  oxide 

64 

100 

661 

Carbon  bisulphide.  . 
Carbon  bisulphide  .... 
Ethyl  ether  

91 
117 
54 

158 
100 
160 

659 
671 
684 

Carbon  dioxide  
Carbon  dioxide  
.Nitrogen  

17 
71 
15 

127 
0 
3 

687 
760 
760 

Ethyl  ether  
Ethyl  ether  

68 
87 

128 
100 

661 
644 

Hydrogen  

4 

3 

760 

TABLE  II. — ADSORPTION  OP  GASES  BY  COCOANUT  CHARCOAL 

Volumes  gas  adsorbed  per  volume  charcoal. 

Pressures  not  given;  volumes  corrected  to  0°  and  760  mm. 

Temperature  in  Celsius  degrees. 


Gas 

Vol. 

Temp. 

Gas 

Vol. 

Temp. 

Argon  

12 

o 

Nitrogen  

15 

0 

Argon 

175 

—  185 

Nitrogen  . 

155 

-185 

Oxygen 

18 

o 

Hydrogen 

4 

0 

Oxygen  

230 

—  185 

Hydrogen  

135 

-185 

Carbon  monoxide. 

21 

0 

Helium             

2 

0 

Carbon  monoxide  .  .  . 

190 

-185 

Helium  

15 

-185 

In  Table  III  are  given  some  more  recent  data  by  Hempel  and 
Vater,1  who  used  a  special  animal  charcoal,  which  they  mixed  to 
1  Zeit.  Elektrochemie,  18,  724  (1912). 


ADSORPTION  OF  GAS  OR  VAPOR  BY  SOLID 


a  stiff  paste  with  ox-blood  diluted  tenfold  with  water  and  which 
they  then  charred  at  600°. 

TABLE  III.— ADSORPTION  OP  GASES  BY  SYNTHETIC  CHARCOAL 

Volumes  gas  adsorbed  per  volume  synthetic  600°  charcoal. 
Volumes  probably  corrected  to  0°  and  760  mm. 


+  20° 

-78° 

-185° 

+  20° 

H2 

7  3 

19.5 

284.7 

H2S 

213  0 

N2  

21.0 

107.4 

632.2 

C12 

304.5 

CO              

26.8 

139.4 

697.0 

NH3 

197.0 

O2                       ....... 

25  4 

122  4 

SO2 

337  8 

+  20° 

-78° 

+  20° 

-78° 

Ar              

12.6 

92.6 

CH4 

41.7 

174.3 

NO 

103  6 

231.3 

C2H4 

139.2 

360  7 

N2O     

109.4 

330.1 

C2H6 

119.1 

275.5 

CO2             

83.8 

568.4 

C2H2 

135.8 

488.5 

TABLE  IV. — BOILING-POINTS  OF  VARIOUS    LIQUIDS 


Liquid 

B.  Pt. 

Liquid 

B.  Pt. 

Liquid 

B.  Pt. 

He. 

-268  7 

C2H6 

—84 

CH3CHO 

+20  8 

H2 

—  252  6 

HCl 

-82  9 

(C2H6)20 

+34  6 

N2  

—  195  7 

C2H2 

-82  41 

CS2 

+46  2 

CO 

-190 

CO2 

-791 

Br2 

+  58  6 

Ar  

-186.1 

H2S 

-60  2 

CHC13 

+61  2 

02  
CH4 

-182.8 
-164 

C12 
NH3 

-33.7 
-33  5 

CH3OH 
CC14 

+64.7 
+  76  7 

NO  

-153 

(CN)a 

-21. 

C2H5OH 

+78  4 

C2H4  

-105 

SO2 

—  10 

C6H6 

+80  2 

N2O  

-  89.8 

H2O 

+  100.0 

For  purposes  of  reference  there  are  given  in  Table  IV  the  boil- 
ing-points of  some  of  the  substances  whose  adsorption  by  carbon 
has  been  studied.  As  a  first  approximation  it  is  often  stated  that 
a  gas  or  vapor  is  adsorbed  more  readily  the  higher  its  boiling- 
point.  Thus  helium  is  taken  up  by  charcoal  much  less  than  hy- 
drogen, and  hydrogen  again  is  adsorbed  to  a.  much  less  extent 

1  Sublimation  point. 


6 


APPLIED  COLLOID  CHEMISTRY 


than  nitrogen  or  oxygen.  Carbon  dioxide  is  adsorbed  less 
readily  than  ammonia,  so  that  these  substances  follow  the  em- 
pirical rule.  Argon,  however,  is  adsorbed  less  completely  by 
charcoal  than  is  nitrogen,  while  carbon  monoxide  is  adsorbed 
to  a  greater  extent  at  0°  than  either  argon  or  oxygen,  though 
according  to  the  rule  this  ought  not  to  be  so.  Nitrous  oxide  is 
adsorbed  less  strongly  than  ethylene,  and  nitric  oxide  more 
strongly  than  methane,  which  is  not  according  to  the  boiling- 
points.  Ethane,  ethylene,  and  acetylene  are  adsorbed  more  at 
+20°  than  is  carbon  dioxide,  though  the  last  is  the  most  readily 
condensible  gas  of  the  four.  The  difference  between  carbon  di- 
oxide and  hydrogen  sulphide  is  in  the  right  direction;  but  seems 
out  of  all  proportion  to  the  difference  in  boiling-points.  Hydrogen 
sulphide  is  adsorbed  more  than  ammonia,  which  is  wrong;  and 
chlorine  is  adsorbed  much  more  strongly  than  ammonia,  although 
the  two  boiling-points  are  practically  identical.  Cyanogen  is  ad- 
sorbed more  than  ammonia  at  70°  and  less  at  0°.  In  the  case  of 
vapors  there  is  no  relation  between  boiling-point  and  adsorption. 
Going  from  higher  to  lower  boiling-points,  we  have  the  order: 
water,  benzene,  ethyl  alcohol,  carbon  tetrachloride,  methyl 
alcohol,  chloroform,  ether,  and  aldehyde.  The  order  from  greater 
to  lesser  adsorption  is  methyl  alcohol,  ethyl  alcohol,  aldehyde, 
ether,  benzene,  water,  chloroform,  and  carbon  tetrachloride.  The 
truth  of  the  matter  is  that  adsorption  is  specific  and  varies  with 
the  nature  of  the  gas  and  of  the  adsorbing  solid.  One  charcoal 
is  not  like  another.  In  Table  V  are  Hunter's  results  on  charcoal 

TABLE  V. — ADSORPTION  OF  GASES  BY  CHARCOAL  FROM  DIFFERENT  WOODS 

Volumes  gas  adsorbed  per  volume  charcoal. 
Temperature  =  0°C;  pressure  =  760  mm. 


Wood 

Vol. 
NH» 

Vol. 
C02 

Vol. 

(CN). 

Wood 

Vol. 
NH6 

Vol. 
C02 

Vol. 

(CN)2 

Logwood  
Ebony.  .    .    . 

111 

107 

55 

47 

87 
90 

Jamaica  logwood  .... 
Sapan  wood 

69 
70 

33 
32 

Camwood  

91 

45 

Beech  

58 

Green  ebony  

90 

41 

Rosewood  . 

51 

Fustic  (Cuba)  .... 
Lignum  vitse  
Boxwood  

90 
89 
86 

58 
47 
31 

29 

Wistaria  sinensis  .... 
Vegetable  ivory  
Cocoanut  shell  

44 
176 

50 
71 

57 
114 

ADSORPTION  OF  GAS  OR  VAPOR  BY  SOLID  7 

made  from  different  woods.  The  cocoanut  charcoal  has  the 
greatest  adsorbing  power  of  all  and,  of  the  others,  logwood  char- 
coal has  the  greatest  adsorbing  action  on  ammonia,  fustic  charcoal 
on  carbon  dioxide,  and  ebony  charcoal  on  cyanogen.  It  is  not  the 
most  porous  charcoal  in  the  ordinary  sense  which  adsorbs  the 
most  gas,  because  Hunter  says  that  the  charcoal  made  from  the 
shell  of  the  cocoanut  is  very  dense  and  brittle.  The  pores  are 
quite  invisible  and,  when  the  charcoal  is  broken,  the  fracture 
presents  a  semi-metallic  lustre.  It  has  been  suggested  by  De 
Saussure1  that  charcoals  with  very  many  pores  of  very  small 
diameter  are  the  best  adsorbers  because  they  present  the  larg- 
est surfaces  for  condensation.  This  is  not  quite  right  because 
there  are  other  factors  besides  the  extent  of  surface,  among  them 
the  arrangement  of  the  surfaces.  A  plane  surface  will  adsorb 
a  given  amount  of  a  gas  under  standard  conditions,  but  two 
plane  surfaces  placed  close  to  each  other  will  apparently  adsorb 
a  good  deal  more  gas  than  the  same  surfaces  would  if  each  did  not 
re-enforce  the  other  and  modify  the  concentration  gradient.  The 
adsorption  depends  not  only  on  the  surface  but  on  the  structure 
of  the  the  adsorbing  material.  Narrow  pores,  down  to  a  cer- 
tain limit,  should  adsorb  more  than  spherical  pores  of  large  dia- 
meter and  this  actually  happens.  Table  VI  shows  the  change  in 
the  adsorption  of  air2  when  different  charcoals  are  heated  from 
-100°  to  +500°.  In  a  number  of  cases  the  parallelism  between 
density  and  adsorption  holds  but  by  no  means  in  all  cases. 
While  there  may  be  an  actual  difference  in  the  amount  of  sur- 
face, we  are  undoubtedly  dealing  with  another  factor,  the  pres- 
ence in  the  charcoal  of  substances  which  affect  the  adsorption. 
Charcoal  is  far  from  being  pure  carbon  and  the  great  improve- 
ments in  charcoal  as  used  in  warfare  have  been  along  the  line  of 
cleaning  the  charcoal  very  thoroughly  in  addition  to  increasing 
the  surface.  While  cocoanut  charcoal  is  usually  considered  the 
best,  the  sour  cherry  charcoal  in  Table  VI  does  better;  and  the 
synthetic  charcoal  of  Hempel  and  Vater3  was  better  than  the 
best  cocoanut  charcoal  which  could  then  be  obtained,  though 
far  inferior  to  the  best  cocoanut  charcoal  of  today.  The  varying 

1  Cf.  DEWAR:  Proc.  Roy.  Inst.,  18,  180  (1905). 

2  PIUTTI  and  MAGLIA:  Gazz.  chim.  ital.,  40  I,  569  (1910). 

3  Zeit.  Elektrochemie,  18,  724  (1912). 


8 


APPLIED  COLLOID  CHEMISTRY 


behavior  of  charcoal  from  cocoanut  shell,  logwood,  fustic,  and 
ebony  is  probably  due  to  the  presence  of  different  impurities 
which  affect  different  gases  in  different  ways. 


TABLE  VI. — BEHAVIOR  OF  DIFFERENT  CHARCOALS 


Apparent 
density 

cc  gas  per  cc 
charcoal  —  100° 
to  +550° 

Air 

Cli 

Prunus  cerasus  
Cocus  nucif  era  
Phillyrea  media  

Sour  cherry1  

1.6393 
1.4497 
1.4970 
1.4383 
1.1901 
1.3000 
1.4192 
1.3398 
1.2802 
1  .  2359 
1.6274 
1.3994 
1.3132 
1.3114 
1.2218 

280.1 
251.8 

222.4 
212.7 
199.5 
186.3 
138.5 
134.5 
134.1 
133.0 
131.5 
129.5 
118.6 
111.1 
99.9 

312.7 
334  .  2 
260.0 

Cocoanut1  

Jasmine  box  
Silk  oak  
White  spruce 

Grevillea  robusta  
Abies  alba 

Castanea  vesca  
Buxus  sempervirens  .  .  . 
Liospunos  kaki  
Populus  nigra  

European  chestnut  .  . 
Common  box  
Chinese  persimmon  .  . 
Black  poplar  

Prunus  virginiana  
Ceratonia  siliqua  
Prunus  armeniaca  
Juglans  regia  
Pinus  rigida.  .  .  
Robinia  pseudo-acacia  . 

Choke-cherry  
Carob1 

Apricot1  
English  walnut  
Pitch-pine  

Common  locust  

Charcoal  is  used  in  most  gas  masks,  because  it  is  the  best  all- 
round  adsorbent  for  toxic  gases.2  The  removal  of  the  toxic 
materials  must  be  surprisingly  complete.  The  adsorbent  must 
be  capable  of  reducing  the  concentration  of  the  toxic  gas  from, 
say  1000  p. p.m.3  to  1  p.p.m.  or  less  within  the  0.1  second  that  the 
air  takes  in  passing  through  the  canister.  This  is  accomplished 
with  a  safe  margin  by  the  present  gas  mask  materials.  In  fact 
it  has  been  shown  that  charcoal  will  reduce  a  concentration  of 
7000  p.p.m.  of  chloropicrin,  CC13NO2,  in  a  rapidly  moving  cur- 
rent of  air  to  less  than  0.5  p.p.m.  in  something  under  0.3  seconds. 
Some  use  has  been  made  in  peace  times  of  the  fact  that  properly 

1  Shells  and  kernels. 

2  LAMB,  WILSON  and  CHANEY:  Jour.  Ind.  Eng.  Chem.,  11,  420  (1919). 

3  Parts  per  million  by  volume. 


ADSORPTION  OF  GAS  OR  VAPOR  BY  SOLID  9 

prepared  charcoal  will  adsorb  gases  to  a  considerable  extent. 
Melsens1  placed  wood  charcoal,  saturated  with  dry  chlorine,  in 
the  longer  branch  of  a  bent  sealed  tube  and  dipped  that  end  of 
the  tube  into  boiling  water,  while  the  shorter  branch  was  placed 
in  a  freezing  mixture.  A  portion  of  the  gas  was  volatilized  and 
the  pressure  thus  developed  caused  liquefied  chlorine  to  appear 
in  the  cold  portion  of  the  tube.  Sulphur  dioxide,  hydrogen 
sulphide,  hydrobromic  acid,  and  cyanogen  can  be  liquefied  in 
the  same  way;  but  not  bromine,  hydrochloric  acid,  carbon  bi- 
sulphide, ether,  or  alcohol. 

In  1874  Tait  and  Dewar2  described  the  use  of  charcoal  as  a 
means  of  getting  a  high  vacuum.  They  placed  a  piece  of  char- 
coal in  a  tube  having  platinum  electrodes,  heated  the  tube  to  a 
red  heat,  exhausted  it  with  a  Sprengel  pump  and  sealed  it.  If 
the  tube  was  cool,  no  spark  would  pass  when  using  an  induction 
coil  capable  of  giving  a  quarter  of  an  inch  spark.  On  heating 
the  tube  slightly  some  gas  was  given  off  from  the  charcoal  and  the 
spark  passed.  By  making  use  of  the  increased  adsorption  at  low 
temperatures  quite  surprising  results  Were  obtained.3  A  bulb 
of  300  cc  capacity,  containing  air  at  15°  and  an  initial  pressure 
of  1.7  mm,  was  connected  with  another  bulb  holding  five  grams 
of  charcoal.  When  the  charcoal  was  cooled  in  liquid  air,  the 
pressure  dropped  to  0.00005  mm.  A  Crookes  radiometer  tube, 
filled  with  hydrogen  at  atmospheric  pressure,  had  a  charcoal 
bulb  attached.  On  dipping  the  charcoal  bulb  into  liquid  air, 
no  rotation  took  place  even  when  the  beam  from  an  electric  arc 
was  focussed  on  the  vanes.  The  motion  became  quite  rapid 
however  when  the  charcoal  bulb  was  placed  in  liquid  hydrogen 
because  the  adsorption  of  hydrogen  by  charcoal  is  sufficient  at 
these  very  low  temperatures,  though  not  at  the  temperature  of 
liquid  air.  The  adsorption  of  helium  by  charcoal  at  -252°  is 
not  enough  to  make  this  experiment  successful  when  the  radio- 
meter tube  is  filled  with  helium. 

Since  different  gases  are  adsorbed  in  different  amounts,  the 
ratio  of  two  gases  in  the  charcoal  will  differ  from  the  ratio  in  the 
gas  phase  and  a  separation  will  be  possible  to  some  extent. 

1  Comptes  rendiis,  77,  781  (1873);  Jour.  Chem.  Soc.,  27,  120  (1874). 

2  Proc.  Roy.  Soc.  Edin.,  8,  348,  628  (1874). 

3  DEWAR:  Proc.  Roy.  Inst.,  18,  437,  756  (1906). 


10  APPLIED  COLLOID  CHEMISTRY 

Dewar1  heated  about  50  grams  of  charcoal  and  then  saturated 
it  at  -185°  in  a  current  of  pure  dry  air  at  atmospheric  pressure, 
at  76  mm,  and  at  5  mm.  The  vessel  containing  the  charcoal 
was  taken  out  of  the  liquid  air  and  the  temperature  allowed  to 
rise  to  15°.  The  volumes  of  gas  given  off  were  5.7,  4.8,  and  4.5 
liters  respectively  and  the  oxygen  concentration  varied  irregu- 
larly between  56  and  58  percent.  These  experiments  show  the 
practical  constancy  of  the  mean  composition  of  the  gases  ad- 
sorbed by  the  charcoal  under  the  conditions  of  these  experiments 
and  they  therefore  show  that  wide  changes  in  the  pressure  of  the 
air  current  have  but  slight  effect  in  altering  the  relative  propor- 
tions of  the  adsorbed  gases.  In  another  experiment  the  vessel 
containing  the  saturated  charcoal  was  placed  in  a  vacuum  vessel 
containing  a  little  liquid  air  so  that  the  temperature  might  rise 
slowly.  This  made  it  possible  to  collect  and  analyze  separately 
the  successive  liters  of  gas  as  evolved.  The  data  are  given  in 
Table  VII.  The  mean  composition  of  the  six  liters  was  56  per- 

TABLE  VII. — DISTILLATION  OF  AIR  ADSORBED  BY  CHARCOAL 


First  liter  
Second  liter  
Third  liter 

18.5  percent  O2 
20.  6  percent  O  2 
53  0  percent  O2 

Fourth  liter  
Fifth  liter  
Sixth  liter 

72.0  percent  O2 
79.0  percent  O2 
84  0  percent  O2 

cent  oxygen.  A  few  experiments  were  also  made  with  special 
mixtures  of  nitrogen  and  oxygen.  When  a  gas  mixture  contain- 
ing 6.5  percent  oxygen  was  used  and  the  charcoal  was  heated 
rapidly  to  15°,  five  liters  of  gas  were  obtained,  containing  23  per- 
cent oxygen. 

At  higher  temperatures  the  results  seem  to  be  quite  different, 
though  this  may  be  due  to  the  removal  of  oxygen  by  combustion. 
Skey2  found  that  red-hot  charcoal  retains  over  three  volumes  of 
nitrogen  which  is  given  off  when  the  cooled  charcoal  is  immersed 
in  water.  Storey  and  Lewis 3  state  that  it  is  difficult  to  remove 
adsorbed  nitrogen  and  carbon  dioxide  from  coke  even  by  heating 

1  Proc.  Roy.  Soc.,  74,  126  (1904);  Proc.  Roy.  Inst.,  18,  184  (1905). 

2  Chem.  News,  15,  15,  27  (1867). 

3  Am.  Chem.  Jour.,  4,  409  (1883). 


ADSORPTION  OF  GAS  OR  VAPOR  BY  SOLID  11 

the  containing  flask  to  200°  and  exhausting  with  a  Sprengel 
pump.  In  these  two  cases  the  oxygen  was  probably  burned  com- 
pletely. It  is  not  so  easy,  however,  to  account  for  Blumtritt's 
results.1  He  heated  various  forms  of  charcoal  under  mercury 
to  140°  in  a  paraffin  bath  and  analyzed  the  gases  which  were 
given  off.  With  common  wood-charcoal  the  gas  was  pure  ni- 
trogen. After  the  same  charcoal  had  been  moistened  and  dried, 
the  evolved  gas  contained  85.6  percent  N2,  2.1  percent  O2, 
9.2  percent  CO2,  and  3.1  percent  CO.  The  gas  from  poplar 
charcoal  consisted  of  83.5  percent  N2,  and  16.5  percent  CO2, 
that  from  crushed  alder  charcoal  had  the  composition  78.9 
percent  N2  and  21.1  percent  CO2,  while  charcoal  made  from 
ash  yielded  a  gas  consisting  of  76.0  percent  N2,  9.1  percent  CO2, 
and  14.9  percent  02.  These  variations,  if  real,  probably  depend 
on  the  nature  and  amount  of  the  hydrocarbons  left  in  each  char- 
coal; but  the  experiments  should  be  repeated.  Craig2  heated  a 
Jena  flask  filled  with  lignite  coke  tp  redness,  closed  the  flask  and 
analyzed  the  contents.  The  unadsorbed  gas  was  nearly  pure 
nitrogen  and  so  was  the  adsorbed  gas,  which  meant  that  the 
oxygen  had  oxidized  some  unspecified  hydrocarbon  without 
burning  it  to  CO  or  C02. 

In  Dewar's  experiments  the  oxygen  adsorbed  at  —185°  was 
apparently  given  off  at  — 15°  without  any  appreciable  contamina- 
tion with  CO  or  CO2.  Calvert3  observed  no  reaction  at  ordinary 
temperatures,  when  carefully  purified  pieces  of  boxwood  char- 
coal were  heated  to  redness  and,  while  still  red-hot,  were  intro- 
duced into  a  cylinder  standing  over  mercury  and  containing  a 
measured  volume  of  oxygen.  Passing  through  the  mercury 
undoubtedly  cooled  the  charcoal  a  great  deal  before  it  came  in 
contact  with  the  oxygen.  By  special  experiments  Calvert  satis- 
fied himself  that  no  carbonic  acid  had  been  formed,  or  existed, 
in  the  cube  of  charcoal.  Since  the  adsorbed  oxygen  oxidized 
ethyl  alcohol  to  acetic  acid,  and  ethylene  to  carbon  dioxide  and 
water,  it  is  certain  that  there  cannot  have  been  a  serious  error 
in  analysis,  because  such  oxidations  could  not  be  caused  by  carbon 
monoxide  or  carbon  dioxide. 

1  Jour,  prakt.  Chem.,  98,  418  (1886). 

2  Chem.  News,  90,  109  (1904). 

3  Jour.  Chem.  Soc.,  20,  293  (1867). 


12  APPLIED  COLLOID  CHEMISTRY 

Quite  different  results  were  obtained  by  C.  J.  Baker1  working 
with  purified  blood  charcoal  which  was  allowed  to  adsorb  oxy- 
gen at  — 15°  and  was  then  pumped  out.  On  heating  the  tube  to 
100°  seven  volumes  of  CO2  were  obtained  when  moist  oxygen 
was  used,  while  no  gas  was  given  off  when  dry  oxygen  was  used. 
In  a  check  experiment  it  was  shown  that  no  carbon  dioxide  was 
formed  when  water  vapor  and  carbon  were  heated  to  100°  in  a 
sealed  tube  for  a  week.  The  question  arose  whether  dry  oxygen 
was  adsorbed  or  whether  it  was  held  more  firmly  than  moist 
oxygen.  Practically  no  gas  was  given  off  up  to  350°.  At  450°  a 
regular  stream  of  gas  was  given  off  which  consisted  of  99  percent 
CO  and  1  percent  CO2  in  those  cases  where  the  drying  had  not 
been  done  so  carefully.  Intermediate  values  were  found  for 
intermediate  degrees  of  drying.  Rhead  and  Wheeler2  do  not 
confirm  Baker's  results  though  this  may  be  because  they  did 
their  experiments  differently.  Rhead  and  Wheeler  treated 
purified  carbon  at  some  definite  temperature,  200°,  300°,  etc., 
with  air,  pumped  out  the  air,  and  then  heated  the  charcoal  more 
or  less  rapidly  to  1100°  and  analyzed  the  gas.  With  an  initial 
temperature  of  300°  and  rapid  heating  to  1100°  the  composition 
of  the  evolved  gas  was  11.3  percent  CO2  and  88.4  percent  CO 
when  dried  air  was  used,  and  15.4  percent  CO2  plus  84.4  percent 
CO  when  moist  gas  was  used.  When  the  initial  temperature 
was  300°,  the  gas  coming  off  between  300°  and  400°  was  composed 
of  57.0  percent  CO2  and  42.4  percent  CO;  between  400°  and  500° 
the  composition  was  41.6  percent  CO2  and  58.0  percent  CO, 
dropping  to  0.8  percent  C02  and  98.7  percent  CO  for  the  gas 
drawn  off  between  800°  and  1100°.  Baker  found  1  percent  C02 
and  99.0  percent  CO  at  450°;  but  his  initial  temperature  was 
— 12°.  The  two  sets  of  experiments  should  be  repeated  so  that 
we  might  know  whether  they  are  or  are  not  contradictory. 
Neither  Rhead  and  Wheeler  nor  Baker  found  any  oxygen  in  the 
gas  given  off  above  200°.  By  determining  the  oxidizing  action 
at  different  temperatures  of  charcoal  containing  adsorbed  oxy- 
gen, it  should  be  possible  to  determine  approximately  at  what 
temperature  all  the  oxygen  was  converted  into  CO  and  C02. 
In  spite  of  all  the  excellent  work  that  has  been  done  on  the  com- 

1  Jour.  Chem.  Soc.,  61,  249  (1887). 

2  Ibid.,  103,  461,  1210  (1913). 


ADSORPTION  OF  GAS  OR  VAPOR  BY  SOLID  13 

bustion  of  carbon,  there  is  still  a  great  deal  that  we  do  not  know. 
Hulett  has  found  that  there  is  a  slow  reaction  between  oxygen 
and  charcoal  at  ordinary  temperatures,  apparently  the  formation 
of  a  non-volatile  oxide. 

When  one  blows  a  slowly  dying  wood  fire  with  the  bellows,  the 
fire  will  often  start  burning  briskly  again,  in  spite  of  the  fact  that 
the  large  influx  of  cooler  air  must  tend  to  chill  the  embers  and 
thus  to  put  the  fire  out.  Bacon1  suggests  that  the  dying  embers 
are  covered  with  an  adsorbed  film  of  carbon  dioxide  which  is  swept 
away  to  some  extent  by  the  draught  from  the  bellows.  When 
the  fire  is  burning  briskly,  there  is  less  carbon  dioxide  adsorbed 
at  the  higher  temperature  and  the  natural  draught  is  sufficient  to 
bring  oxygen  to  the  burning  wood.  Though  this  hypothesis 
has  not  been  proved,  it  is  rendered  more  probable  by  the  experi- 
ments of  Arndt  and  Schraube,2  who  found  that  even  at  a  red 
heat  carbon  dioxide  is  adsorbed  more  strongly  than  carbon  mon- 
oxide or  nitrogen.  Manville's  fluptuations  of  the  ignition  point 
of  charcoal3  are  probably  due  to  hydrocarbons  which  had  not 
been  removed. 

Ramsay4  has  availed  himself  of  Dewar's  method  of  using 
cooled  cocoanut  charcoal  as  an  adsorbent  for  gases  to  determine 
the  amounts  of  neon  and  helium  in  the  air.  At  100°  the  oxygen, 
argon,  and  nitrogen  are  taken  out  practically  quantitatively 
while  the  adsorption  of  neon  and  helium  is  negligible.  At  — 185° 
practically  all  of  the  neon  is  adsorbed  and  none  of  the  helium. 
From  his  observations,  Ramsay  concludes  that  the  concentra- 
tion of  neon  in  the  air  is  about  0.00123  volume  percent  and  that  of 
helium  about  0.00044  volume  percent,  while  the  amount  of 
free  hydrogen  cannot  be  greater  than  one  five-hundredth  of  the 
combined  volumes  of  the  neon  and  helium.  At  great  elevations 
this  changes  completely,  the  gas  with  the  lowest  molecular 
weight  concentrating  in  the  higher  regions.  Alleman5  gives 
some  data  for  the  estimated  concentrations  of  hydrogen  at 
different  heights  above  sea-level,  Table  VIII. 

1  Jour.  Phys.  Chem.,  17,  768  (1913). 

2  Nernst  Festschrift,  46  (1912). 

3  Jour.  Chim.  phys.,  6,  297  (1907). 

*Proc.  Roy.  Soc.,  76A,  111  (1905);  80A,  599  (1908). 
*  Jour.  Franklin  Inst.,  186,  161  (1918). 


14  APPLIED  COLLOID  CHEMISTRY 

TABLE  VIII. — ESTIMATED  DISTRIBUTION  OF  HYDROGEN  IN  THE  ATMOSPHERE 


Height  in 
kilometers 

Percent 
hydrogen 

Height  in 
kilometers 

Percent 
hydrogen 

Height  in 
kilometers 

Percent 
hydrogen 

140 

99.2 

80 

64.7 

20 

0.04 

130 

99.0 

70 

32.6 

15 

0.02 

120 

98.7 

60 

10.7 

11 

0.01 

110 

98.1 

50 

2.80 

5 

0.01 

100 

95.6 

40 

0.67 

90 

88.3 

30 

0.16 

Dewar1  has  worked  out  a  very  effective  lecture  experiment, 
based  on  the  selective  adsorption  by  charcoal.  A  number  of 
spectroscopic  tubes,  connected  in  series  with  a  large  U-tube  con- 
taining charcoal  are  exhausted  by  cooling  the  charcoal  until  the 
electric  discharge  will  barely  pass.  The  charcoal  tube  is  now 
placed  in  liquid  air  and  a  current  of  air  is  allowed  to  enter  the 
system  slowly,  passing  first  through  the  charcoal  tube  which 
takes  out  the  readily  adsorbed  gases  such  as  oxygen,  nitrogen, 
and  argon,  while  helium,  neon  and  hydrogen  pass  through. 
When  the  pressure  in  the  first  tube  has  risen  sufficiently  it  begins 
to  glow  with  the  well-known  rich  orange  hue  of  neon.  In  time 
the  characteristic  discharge  of  neon  and  helium  appears  in  the 
other  tubes.  The  sensitiveness  of  the  neon  tubes  to  induced 
electric  oscillations  from  a  coil  of  wire  placed  at  right  angles  to 
the  tube  may  be  shown.  Claude2  points  out  that  the  remarkable 
luminosity  of  neon  can  be  utilized  for  lighting  purposes.  One  of 
the  difficulties  of  the  problem  is  the  ease  with  which  neon  is 
masked  by  small  quantities  of  certain  other  gases.  It  is  not 
enough  to  introduce  very  pure  neon  into  a  tube  with  electrodes 
under  suitable  pressure  in  order  to  obtain  an  effectively  luminous 
tube.  This  can  be  overcome  by  removing  these  other  gases 
with  cooled  charcoal,  when  the  beautiful  orange  luminescence  of 
neon  appears,  and  retains  its  brightness.  The  light  is  very  rich 
in  red  rays  and  Claude  considers  it  just  the  corrective  required 
for  the  light  of  mercury  vapor  tubes. 

1  Proc.  Roy.  Inst.,  18,  444  (1906). 

2  Mon.  Sci.,  74,  135;  Jour.  Franklin  Inst.,  171,  624  (1911). 


ADSORPTION  OF  GAS  OR  VAPOR  BY  SOLID  15 

Krypton  and  xenon  may  be  obtained  from  the  air  by  means  of 
cooled  charcoal.1  A  current  of  air  passes  through  a  series  of 
tubes  immersed  in  liquid  air  for  the  purposes  of  purification,  the 
last  tube  containing  cotton  wool  in  order  to  retain  any  dust  of 
the  solid,  condensed  impurities.  This  pure  air  is  passed  through 
a  tube  containing  about  100  grams  of  charcoal  for  at  least  24 
hours.  The  charcoal  tube  is  then  removed  and  placed  in  solid 
carbonic  acid,  and  the  gas  which  is  evolved  is  allowed  to  escape. 
The  gas  remaining  in  the  charcoal  at  —  78°  is  driven  off  by  heat- 
ing and  exhaustion,  and  all  the  carbon  compounds  and  oxygen 
removed  from  it.  The  remaining  gas,  consisting  of  nitrogen, 
krypton,  and  xenon,  is  separated  into  its  constituents  by  conden- 
sation and  fractionation.  Instead  of  passing  a  current  of  air 
over  charcoal  at  — 183°,  a  few  hundred  grams  of  charcoal  may  be 
covered  with  liquid  air  and  the  latter  may  be  allowed  to 
evaporate  in  a  silvered  vacuum  vessel.  The  gases  remaining  in 
the  charcoal  can  then  be  separated  as  before.  In  this  way  spec- 
trum tubes  of  krypton  and  xenon  may  be  readily  prepared. 

Rutherford2  found  that  cocoanut  charcoal  adsorbs  radio-active 
emanations.  A  slow  current  of  air  charged  with  the  emanations 
of  radium,  thorium,  or  actinium  is  deprived  of  some  or  all  of  its 
emanation  in  the  passage  through  a  tube  filled  with  cocoanut 
charcoal,  and  for  this  purpose  the  charcoal  need  only  be  at 
ordinary  temperature.  The  complete  withdrawal  of  the  eman- 
ation only  takes  place  when  the  air  is  passed  very  slowly 
through  the  charcoal;  if  the  speed  is  increased,  the  amount  of 
unadsorbed  emanation  increases  also.  In  another  experiment 
Rutherford  found  that  if  a  tube  containing  less  than  a  gram  of 
charcoal  is  connected  to  a  vessel  containing  the  emanation  from 
several  milligrams  of  radium  bromide,  the  charcoal  will  adsorb 
the  emanation  in  time.  If  some  powdered  willemite  be  mixed 
with  the  charcoal,  the  gradual  adsorption  is  shown  by  the  increas- 
ing brilliancy  of  phosphorescence  of  the  willemite.  It  is  not 
necessary  to  heat  or  exhaust  the  charcoal  previously;  but  if  this 
is  done  the  emanation  is  adsorbed  more  rapidly.  The  charcoal 
retains  the  emanation  at  ordinary  temperature  but  the  greater 

1  VALENTINER   and    SCHMIDT:  Sitzungsber.    Akad.    Wiss.    Berlin,    1905, 
816;  DEWAR:  Proc.  Roy.  Inst.,  18,  445  (1906). 

2  Nature,  74,  634  (1906). 


16  APPLIED  COLLOID  CHEMISTRY 

part  of  it  is  expelled  by  heating  to  a  low  red  heat.  Boyle 1  showed 
that  cocoanut  charcoal  adsorbed  thorium  emanation  the  most 
and  ordinary  charcoal  the  least,  while  animal  charcoal  came  in 
between  the  other  two.  The  rate  of  adsorption  increased  very 
much  when  cocoanut  charcoal  was  used  at  —75.5°  and  only 
slightly  in  the  case  of  wood  charcoal.  At  211°  cocoanut  char- 
coal begins  to  give  off  carbon  dioxide ;  but  it  shows  some  power  of 
adsorbing  thorium  emanation  up  almost  to  300°.  It  is  not  possible 
for  the  charcoal  to  become  saturated  with  thorium  emanation 
as  may  happen  with  other  gases,  because  the  thorium  emanation 
adsorbed  by  the  charcoal  is  decaying  at  the  rate  of  half-value 
in  54  seconds. 

The  general  rule  for  mixtures  of  gases  seems  to  be2  that  the 
more  readily  adsorbed  gas  displaces  the  other  to  some  extent  and 
is  adsorbed  to  a  greater  relative  amount  than  one  would  have 
predicted  from  experiments  on  the  single  gases.  This  cannot  be 
universally  true.  Bergter3  found  that  at  pressures  of  0.5  to  10 
mm  oxygen  is  adsorbed  30  to  40  times  as  strongly  as  nitrogen, 
and  that  at  these  pressures  the  presence  of  oxygen  increases  the 
amount  of  nitrogen  adsorbed.  With  readily  condensible  gases, 
one  may  have  one  of  the  vapors  helping  to  carry  down  the  other. 
Cases  of  this  sort  have  been  studied  by  Hunter,4  the  most  strik- 
ing instance  being  water  and  ammonia.  It  seems  probable  that 
the  adsorbed  water  dissolves  or  adsorbs  ammonia  thus  increas- 
ing the  amount  of  ammonia  apparently  held  by  the  charcoal. 
It  has  been  shown  by  E.  E.  Reid  that  the  presence  of  traces  of 
benzene  or  ether  in  charcoal  will  increase  the  service  time  against 
chloropicrin. 

The  data  in  Table  IX  are  not  tabulated  as  one  would  like  to 
have  them;  but  they  do  indicate  that  the  presence  of  a  second 
gas  will  cut  down  the  adsorption  of  the  first,5  though  such  com- 
parisons should  be  made  at  equal  partial  pressures.  Passing  one 
gas  over  charcoal  which  has  adsorbed  another  gas  should  theo- 

1  Jour.  Phys.  Chem.  12,  284  (1908);  Phil.  Mag.  (6)  17,  374  (1909).     See 
also  SATTERLY:  Phil.  Mag.  (6)  16,  584  (1908);  20,  778  (1910);  PORLEZZA  and 
MORSI:  Atti  Accad.  Lincei,  20  I,  932  (1911). 

2  FREUNDLICH  :  Kapillarchemie,  99  (1909). 

3  Drude's  Ann.  37,  480  (1912). 

4  Jour.  Chem.  Soc.  23,  73  (1870). 

6HEMPEL  and  VATER:  Zeit.  Elektrochemie,  18,  724  (1912). 


ADSORPTION  OF  GAS  OR  VAPOR  BY  SOLID 


17 


retically  cause  complete  displacement  of  the  adsorbed  gas;  but 
it  is  evident  that  the  effectiveness  will  be  much  greater  if  one  dis- 
places the  slightly  adsorbed  gas  by  the  readily  adsorbed  one 
rather  than  the  other  way  round.  It  would  be  very  interesting 
to  determine  an  isotherm  showing  the  relative  amounts  of  two 
gases  in  the  vapor  phase  and  the  charcoal  phase  when  in  equi- 
librium at  constant  pressure. 

TABLE    IX. — ADSORPTION    OP    MIXED    GASES   BY    SYNTHETIC  CHARCOAL 

Mixed  gases  with  0.4  g  synthetic  600°  charcoal. 
Could  adsorb  about  30  cc  ethane. 


cc 
H2 

cc 
C2H6 

Percent 
C2H« 

cc  gas 
adsorbed 

Percent  gas 
adsorbed 

Percent  C2H8 
unadsorbed 

95.0 

5.0 

5 

3.7 

3.7 

1.5 

45.0 

5.0 

10 

3.8 

7.6 

2.5 

20.0 

5.0 

20 

3.8 

15.2 

5.0 

11.7 

5.0 

30 

4.4 

26.4 

7.5 

6.1 

5.0 

45 

4.6 

41.4 

9.5 

cc 
H2 

cc 
CH4 

cc  gas 
adsorbed 

cc 
H2 

cc 
C2H4 

cc  gas 
adsorbed 

95.0 

5.0 

1.3 

95.0 

5.0 

3.6 

15.0 

5.0 

2.7 

15.0 

5.0 

4.4 

6.1 

5.0 

3.3 

6.1 

5.0 

5.1 

2.7 

5.0 

3.9 

2.7 

5.0 

5.5 

cc 
H2 

cc 
C2H2 

cc  gas 
adsorbed 

cc 

N2 

cc 
C2H4 

cc  gas 
adsorbed 

95.0 

5.0 

3.6 

95.0 

5.0 

6.2 

15.0 

5.0 

4.7 

30.0 

5.0 

9.6 

6.1 

5.0 

5.2 

12.2 

5.0 

10.7 

2.7 

5.0 

5.7 

5.4 

5.0 

11.6 

A  curious  case  has  been  cited  by  Mat  win.1  Charcoal  will  take 
carbon  bisulphide  and  carbonyl  sulphide  out  of  illuminating  gas, 
one  kilogram  of  charcoal  cutting  the  sulphur  content  of  ten  cubic 
meters  of  gas  to  2.92  grams.  Porous  charcoals  are  the  best, 


1  Jour.  Gasbeleuchtung,  52,  602  (1909). 

2 


18  APPLIED  COLLOID  CHEMISTRY 

such  as  pine  and  linden.  Bone-black  takes  up  almost  no  carbon 
bisulphide,  and  cocoanut  charcoal  is  said  to  be  even  less  effective. 
This  is  very  remarkable,  because  cocoanut  charcoal  adsorbs 
carbon  bisulphide  strongly.  We  must  either  assume  that  the 
illuminating  gas  cuts  down  the  adsorption  of  carbon  bisulphide 
very  much  or  that  there  is  an  error  in  the  statement.  If  carbon 
bisulphide  and  illuminating  gas  were  adsorbed  in  the  same  ratio 
in  which  they  occur  in  the  mixture,  there  would  be  no  apparent 
purification  even  though  the  actual  adsorption  were  very  large.1 
This  point  calls  for  more  study. 

The  adsorption  of  gases  by  charcoal  has  been  studied  exten- 
sively, because  charcoal  adsorbs  so  many  gases  and  adsorbs  them 
so  strongly.  Experiments  have  also  been  made  with  many 
other  solids — silica,  alumina,  glass,  wool,  rubber,  celluloid, 
meerschaum,  metals,  soils,  etc.  The  behavior  of  glass  is  impor- 
tant because  of  its  use  in  the  laboratory,  Miilfarth2  states  that  the 
adsorption  decreases  in  the  order:  NH3,  SO2,  N2O,  C2H2.  Kundt 
and  Warburg3  found  that  a  small  amount  of  water  is  retained 
obstinately  by  glass  even  when  the  pressure  is  reduced 
practically  to  zero.  In  order  to  remove  this  water  the  glass 
must  be  heated,  in  which  case  one  is  likely  to  cause  changes  in 
the  glass,  setting  free  water  vapor  and  gases  that  were  not  ad- 
sorbed in  any  proper  sense  of  the  term.4  On  the  other  hand, 
the  film  of  water  which  usually  covers  glass  surfaces  is  due  chiefly 
to  alkali  dissolved  from  the  glass.5  It  is  hard  to  tell  whether  the 
change  in  the  vacuum  of  a  Crookes'  tube  is  due  to  adsorption  of 
the  gas  by  the  glass  or  not.  Swinton6  found  that  the  inner 
surfaces  of  these  tubes  are  roughened  perceptibly  after  being 
subjected  to  cathode  ray  bombardment.  When  these  portions 
of  the  tube  are  heated  strongly  in  a  blow-pipe,  they  immediately 
become  clouded,  owing  to  the  formation  of  minute  spherical 
bubbles  which  average  about  0.01  mm  in  diameter.  The  centers 

1  Cf.  LEIGHTON:  Jour.  Phys.  Chem.,  20,  32  (1916). 

2  Drude's  Ann.,  3,  328  (1900). 

3  Wied.  Ann.,  24,  327  (1884);  cf.  BTJKSEN:  Pogg.  Ann.,  156,  201  (1875). 

4  SHERWOOD:  Jour.  Am.  Chem.  Soc.,  40,  1645;  Phys.  Rev.  (2)  12,  448 
(1918). 

5  WARBURG  and  IHMORI:  Wied.  Ann.,  27,  481  (1886);  IHMORI:  Ibid.,  31, 
1006  (1887;. 

6  Proc.  Roy.  Soc.,  79A,  134  (1907). 


ADSORPTION  OF  GAS  OR  VAPOR  BY  SOLID  19 

of  the  bubbles  are  about  0.12  mm  from  the  inner  surface  of  the 
glass.  The  number  of  bubbles  per  square  centimeter  of  glass 
was  found  to  be  about  625,000,  or  about  0.000133  cc  of  gas  per 
square  centimeter.  The  gas  is  mostly  hydrogen  and  is  given  off 
when  the  glass  is  powdered.  It  is  not  known  whether  the  bubbles 
are  connected  with  the  inner  part  of  the  tube  by  minute  channels 
or  not.  If  they  are,  this  is  a  case  of  adsorption.  If  the  bubbles 
are  actually  buried  in  the  glass  owing  to  the  latter  flowing  round 
them,  that  is  not  adsorption. 

L.  J.  Briggs1  showed  that  finely  divided  quartz  adsorbs  water 
vapor,  though  not  to  the  same  extent  as  amorphous  silica. 
About  9  mg  water  was  adsorbed  at  30°  by  50  g  quartz  having 
an  estimated  surface  of  20,000  cm2  when  the  vapor  pressure  was 
26.1  mm,  about  4.5  mg  when  the  vapor  pressure  was  19.6  mm, 
and  about  0.5  mg  when  the  vapor  pressure  was  0.2  mm.  Cohn- 
staedt2  states  that  the  gas  given  off  by  aluminum  electrodes  in  a 
Crookes'  tube  is  water  vapor  and  not  hydrogen,  as  usually  be- 
lieved. He  claims  to  prove  that  glass  and  aluminum  surfaces 
will  retain  water  even  when  heated  to  500°  or  when  dried  over 
phosphorus  pentoxide.  It  is  the  presence  of  a  water  film  of 
varying  thickness  on  a  glass  or  other  vessel  which  makes  it 
necessary  to  keep  the  moisture  constant  if  accurate  weighings  are 
to  be  made.  Professor  Morley  once  said  that  it  was  a  mistake  to 
believe  that  it  is  a  difficult  matter  to  dry  a  gas.  That  is  a  very 
simple  thing  to  do;  but  the  difficulty  is  to  dry  the  vessel  which 
holds  the  gas. 

Drucker  and  Ullmann3  showed  that  the  adsorption  of  a  vapor 
by  the  glass  containing  vessel  causes  only  a  negligible  error  in 
vapor-density  determinations  with  ether,  alcohol,  benzene,  and 
chloroform;  but  the  xerror  may  amount  to  2  percent  with 
acetic  acid  at  80°.  At  higher  temperatures  the  adsorption 
decreases  and  the  error  is  considerably  less.  The  adsorption  of 
ammonia  by  solid  ammonium  hydrosulphide  introduces  a  serious 
error  in  the  equilibrium  relations  for  ammonia  and  hydrogen 
sulphide.4 

1  Jour.  Phys.  Chem.,  9,  617  (1905). 

2  Drude's  Ann.,  38,  223  (1912). 

3  Zeit.  phys.  Chem.,  74,  567  (1910). 

4  MAGNUSSON:  Jour.  Phys.  Chem.,  11,  21  (1907). 


20  APPLIED  COLLOID  CHEMISTRY 

While  most  solids,  even  though  porous,  do  not  adsorb  gases  as 
much  as  charcoal,  yet  Dewar1  reports  that  dry  aluminum  oxide 
shows  a  remarkable  power  of  adsorbing  air  at  low  temperatures, 
one  gram  condensing  some  70  cc  at  atmospheric  pressure.  The 
adsorption  by  alumina  decreases  very,  rapidly  with  decreasing 
pressure,  and  consequently  alumina  cannot  be  used  as  a  means 
of  obtaining  high  vacua.  Meerschaum  and  silica  behave  some- 
what like  alumina.  On  the  other  hand,  high  vacua  can  be  obtained 
by  means  of  copper  powder.2  Carbon  dioxide3  and  water  vapor4 
are  both  adsorbed  strongly  by  alumina.  Platinum  black  may 
take  up  100  volumes  of  hydrogen  and  palladium  nearly  up  to 
3000  volumes  of  hydrogen,5  but  it  is  not  certain  whether  this 
hydrogen  is  all  adsorbed  or  is  partly  dissolved.  In  gas  analysis 
palladium,  heated  to  100°,  is  used  to  remove  hydrogen.  This 
is  purely  a  question  of  rate  because  equilibrium  is  reached  more 
rapidly  at  the  higher  temperatures.  Paal  and  Hohenegger6 
report  on  the  adsorption  of  acetylene  by  palladium.  Platinum 
black  will  take  up  more  than  800  volumes  of  oxygen;  but  it  seems 
certain  that  some  of  the  platinum  has  been  oxidized.7  Celluloid 
adsorbs  carbon  dioxide8  and  rubber  apparently  adsorbs9  it  and 
also  other  gases,  though  it  is  possible  that  we  have  true  solution 
in  the  case  of  rubber.  Wool10  adsorbs  both  hydrochloric  acid 
and  ammonia  gas.  Rohland11  reports  that  unsaturated  com- 
pounds are  adsorbed  by  clay  and  oxygenated  compounds  by  the 
oxides  of  silicon,  aluminum,  iron  and  titanium. 

If  the  film  of  condensed  air  on  a  small  particle  remains  of  the 
same  thickness  irrespective  of  the  diameter  of  the  particle  or  if 
it  does  not  decrease  too  rapidly  with  decreasing  diameter,  the 
ratio  of  air  film  to  mass  of  particle  will  increase  as  the  particle 

1  Proc.  Roy.  Inst.,  18,  435  (1906). 

2  MERTON:  Jour.  Chem.  See.,  106,  645  (1905). 
"SCHEERMESSER:  Centralblatt  (3)  1,  165  (1870). 

4  JOHNSON:  Jour.  Am.  Chem.  Soc.,  34,  911  (1912). 

6  PAAL  and  GERUM:  Ber.  deutsch.  chem.  Ges.,  41,  805  (1908). 

6  Ber.  deutsch.  chem.  Ges.,  43,  2684,  2692  (1910). 

7  ENGLER  and  WOHLER:  Zeit.  anorg.  Chem.,  29  (1909). 
SLEFEBURB:  Jour.  Chem.  Soc.  105,  328  (1914). 

9  REYCHLER:  Van  Bemmelen  Gedenkboek,  55  (.1910). 

10  FREUNDLICH  :  Kapillarchemie,  106  (1909). 

11  Zeit.  anorg.  Chem.,  66,  108  (1909). 


ADSORPTION  OF  GAS  OR  VAPOR  BY  SOLID  21 

grows  smaller.  Cushman  and  Coggeshall1  found  that  a  rock 
powder  which  would  pass  through  a  200  mesh  sieve  surged  like 
a  liquid.  When  poured  into  a  vessel,  it  only  filled  46  percent 
of  the  space,  while  a  coarser  powder  filled  more.  It  is  not  sur- 
prising that  the  air-cushion  around  each  particle  makes  the 
grains  move  one  over  another  like  a  liquid.  A  similar  phenome- 
non has  been  observed  with  ground  phosphate  rock.  With  a 
substance  like  carbon  black,  which  adsorbs  gases  very  markedly, 
as  little  as  5  percent  of  the  apparent  volume  may  be  due  to  the 
carbon  black2  and  a  liter  of  carbon  black  may  contain  2.5  liters 
of  air.3  When  indigo  is  reduced  to  a  very  fine  powder  by  means 
of  a  disintegrator4  the  single  particles  appear  to  be  separated 
one  from  another  by  an  envelope  of  air,  so  that  the  dry,  solid 
powder  occupies  a  space  equal  to  that  of  a  20  percent  paste  of 
indigo,  containing  the  same  amount  of  indigo.  Stoney5  points 
out  that  the  freedom  of  motion  of  finely  divided  solid  particles 
becomes  more  marked  when  there  is  a  temperature  gradient.  He 
cites  the  mobility  imparted  to  very  fine  powders  such  as  magne- 
sium carbonate  or  precipitated  silica  by  heating  them  in  a  metal 
dish.  When  the  dish  is  disturbed  the  powder  glides  about  as  if 
floating  whenever  the  powder  is  able  by  radiation  to  maintain  a 
lower  temperature  than  the  dish. 

Ives,  Kingsbury  and  Karrer6  state  that  the  characteristics  of 
thermocouples  of  varying  diameter  can  be  accounted  for  rather 
satisfactorily  if  it  is  assumed  that  they  may  be  considered  as 
cylindrical  wires  to  which  heat  is  conveyed  through  thin  films 
of  hot  gas,  of  thickness  independent  of  the  temperature  and  size 
of  the  wire. 

The  presence  of  a  film  of  air  or  vapor  round  a  solid  particle 
is  shown  in  a  striking  way  by  the  fact  that  it  is  quite  a  simple 
matter7  to  pass  suspended  phosphorus  pentoxide  through  several 
wash-bottles  in  spite  of  its  tremendously  hygroscopic  nature. 

1  Jour.  Franklin  Inst.,  174,  672  (1912). 

2  CABOT:  Eighth  Int.  Congress  Applied  Chem.  12,  18  (1912). 

3  SABIN:  Technology  of  Paint  and  Varnish,  201  (1917). 

4  Jour.  Soc.  Dyers  and  Colourists,  17,  294  (1901). 
6  Phil.  Mag.  (5)  4,  443  (1877). 

6  Jour.  Franklin  Inst.,  186,  421  (1918). 

?ENGLER  and  WILD:  Ber.  deutsch.  chem.  Ges.,  29,  1929  (1896). 


22  APPLIED  COLLOID  CHEMISTRY 

Tyndall1  found  it  very  difficult  to  get  experimental  tubes  which 
were  absolutely  clean  on  the  surface  and  which  contained  no 
suspended  particles.  Dust-motes  from  the  external  air  passed 
through  U-tubes  containing  potash  and  sulphuric  acid  and 
through  flasks  containing  ethers  or  alcohols.  He  states  that  it 
requires  long-continued  action  on  the  part  of  an  acid,  first  to 
wet  the  motes  and  then  to  destroy  them.2  The  difficulty  in 
stopping  smokes  in  gas  warfare  is  that  the  particles  do  not  come 
in  contact  with  the  materials  in  the  mask. 

Schloesing3  has  put  the  thing  generally  by  saying  that  it  is 
much  easier  to  take  gas  out  of  a  gaseous  mixture  than  to  remove 
suspended  liquid  or  solid  particles.  If  the  sheath  of  adsorbed  air 
or  gas  is  removed,  wetting  may  take  place.  Since  the  sulphur 
trioxide  from  the  contact  process  can  be  caught  in  concentrated 
acid  and  not  satisfactorily  in  water,  the  concentrated  acid  must 
displace  the  gas  film  rapidly  and  fairly  completely,  while  water 
does  not;  but  we  do  not  yet  know  the  reason  for  this.  One  can- 
not agree  with  Knietsch4  however  when  he  says  that  it  is  a  very 
remarkable  and  not  satisfactorily  explained  fact  that  the  sulphuric 
acid  fog  from  the  burner  gases  is  more  difficult  to  precipitate 
when  it  is  cooled  rapidly  than  when  it  is  cooled  slowly.  Rapid 
cooling  gives  smaller  particles  and  the  air-cushion  is  larger 
relatively  to  the  mass;  consequently  precipitation  is  more  difficult. 

Schuster5  studied  the  disruptive  discharge  through  gases  and 
found  an  apparent  diminution  of  dielectric  strength  with  diminu- 
tion of  pressure.  Since  no  sufficient  decrease  in  inductive  capac- 
ity could  be  observed  in  the  mass  of  the  gas,  he  assumed  that 
layer  in  contact  with  the  electrode  there  was  a  condensed  surface 
of  gas  having  a  large  inductive  capacity.  Schuster  pointed  out 
that  some  of  the  most  puzzling  facts  of  the  disruptive  dis-^ 
charge  admit  of  explanation  if  we  once  admit  the  possibility  of 
such  a  contact  layer,  diminishing  in  density  with  decreasing  gas 
pressure.  If  this  layer  of  adsorbed  gas  offers  an  increased  re- 
sistance to  the  passage  of  an  electrical  discharge,  it  follows,  from 

1  Phil.  Trans.,  160,  337  (1870). 

2  See  also  EHRENBERG:  Die  Bodenkolloide,  226  (1915). 

3  Comptes  rendus,  94,  1187  (1882). 

4Ber.  deutsch.,  chem.  Ges.,  34,  4081  (1901). 
6  Phil.  Mag.  (5)  29,  197  (1880). 


ADSORPTION  OF  GAS  OR  VAPOR  BY  SOLID  23 

the  Theorem  of  LeChatelier,  that  an  electrical  stress  will  tend  to 
remove  the  film  of  adsorbed  gas.  This  enables  us  to  account  for 
many  apparently  unrelated  facts.  Electrical  waves  will  tend 
to  remove  active  oxygen  or  active  hydrogen  from  an  electrode 
and  will  therefore  cut  down  the  over-  voltage.1  Superposing  an 
alternating  current  on  a  direct  current  also  decreases  the  over- 
voltage,  making  other  reactions  possible.2  The  experiments  of 
Margules3  and  Ruer4  on  the  dissolving  of  platinum  find  their 
explanation  in  the  cutting  down  of  the  over-voltage.5  With 
direct  current  there  is  oxidation  to  a  higher  and  insoluble  stage. 
With  alternating  current  the  over-voltage  is  decreased  and  little 
or  none  of  the  insoluble  compound  is  formed.  The  electrolytic 
detector,  the  crystal  detector,  and  the  coherer,  as  used  in  wire- 
less telegraphy,  seem  to  owe  their  action  to  the  partial  or  com- 
plete elimination  of  an  air  film  by  means  of  electrical  stress. 
The  essential  difference  between  the  coherer  and  the  detector  is 
that  coalescence  takes  place  readily  in  the  former  case  and  not 
in  the  latter. 

It  is  interesting  to  note  that  Lenard6  found  that  mercury  wets 
platinum  only  when  a  current  is  flowing.  At  other  times  there  is 
evidently  an  air  film.  It  has  been  claimed  by  Allen7  that  the 
photo-electric  fatigue  of  metals  is  due  to  a  change  in  the  surface 
film  or  gas  or  in  the  gas  included  in  the  metal  (Hallwachs). 
It  cannot  be  due  to  a  chemical  change  such  as  oxidation;  to  a 
physical  change  such  as  roughening  of  the  surface;  to  anelectrical 
change  in  the  formation  of  an  electrical  double  layer  (Lenard)  ; 
or  to  a  disintegration  of  the  metal  due  to  the  expulsion  of  electrons 
by  light  (Ramsay  and  Spencer).  The  difference  in  the  results 


:  Drude's  Ann.,   15,   193   (1904);   BENNEWITZ:  Zeit.  phys. 
Chem.,  72,  223  (1910). 

2  ARCHIBALD  and  VON  WARTENBERG:  Zeit.  Elektrochemie,  17,  812  (1911); 
REITLINGER:  Ibid.,  20,  261   (1914);  GHOSH:  Jour.  Am.   Chem.  Soc.,  37, 
33  (1915);  STEPANOFF:  Chem.  Abstracts,  10,  2431  (1916). 

3  Wied.  Ann.,  66,  629;  66,  540  (1898). 

4  Zeit.  phys.  Chem.,  44,  81,  Zeit.  Elektrochemie,  9,  235  (1903);  11,  10, 
661  (1905);  HABER:  Zeit  anorg.  Chem.,  61,  365  (1906). 

6  REITLINGER:  Zeit.  Elektrochemie,  20,  261  (1914). 

6  Wied.  Ann.,  30,  212  (1887). 

7  Phil.  Mag.  (6)  20,  564  (1910);  cf.  LENARD:  Drude's  Ann.,  8,  196  (1902); 
T2.490  (1903). 


24 


APPLIED  COLLOID  CHEMISTRY 


obtained  by  Allen  may  be  due  to  the  fact  that  Allen  worked  at 
atmospheric  pressure,  while  Robinson's  measurements  were 
made  on  electrodes  at  a  high  vacuum.1 

n  cases  of  marked  adsorption  of  gases  it  is  interesting  to 
determine  what  is  the  probable  density  of  the  adsorbed  gas. 
Mitscherlich2  calculated  that,  when  carbon  dioxide  at  atmos- 
pheric pressure  and  12°  is  adsorbed  by  boxwood  charcoal,  the 
carbon  dioxide  occupies  only  one  fifty-sixth  of  its  original  volume. 
Since  this  is  a  lesser  volume  than  the  same  amount  of  carbon 
dioxide  can  occupy  as  a  gas  at  this  temperature,  it  is  usually 
assumed  that  some  of  it  has  been  liquefied.  It  is  possible,  how- 
ever, to  consider  the  carbon  dioxide  as  present  in  the  form  of  a 
highly  condensed  gas  which  has  not  liquefied.  It  might  be  inter- 
esting to  study  the  behavior  of  carbon  dioxide  near  the  critical 
point  in  presence  of  charcoal.  The  magnitude  of  the  heat  of 
adsorption  is  an  argument  in  favor  of  the  actual  liquefaction  of 
the  gas.  Dewar3  has  calculated  the  apparent  density  of  some 
gases  adsorbed  by  cocoanut  charcoal  at  low  temperatures.  The 
data  in  Table  X  show  that  the  densities  are  of  the  same  order 
as  those  of  the  liquid  gases  and  are  greater  in  some  cases.  At 
higher  temperatures  the  amount  of  adsorption  would  be  smaller 
and  the  apparent  condensation  less. 


TABLE  X. — DENSITY  OF  GASES  ADSORBED  BY  CHARCOAL 
Density  in  grams  per  cubic  centimeter. 


Gas  adsorbed 

Temperature 
of  adsorp- 
tion 

Density  of 
adsorbed 
gas 

Density  of 
liquefied 
gas 

Carbon  dioxide 

+  15 

0  70 

0.80 

Oxygen 

-183 

1  33 

1    12 

Nitrogen  

-193 

1.00 

0.84 

Hydrogen 

-193 

0.06 

0.07 

Hydrogen 

—210 

0  08 

Hydrogen  
Helium  

-252 

-258 

0.11 
0.17 

(0.122) 

1  Phil.  Mag.  (6)  23,  255  (1912). 

2  Sitzungsber.  Akad.  Wiss.  Berlin,  1841,  376. 

3  Proc.  Roy.  Inst.,  18,  438  (1906). 


ADSORPTION  OF  GAS  OR  VAPOR  BY  SOLID  25 

It  is  to  be  noticed  that  the  vapor  pressure  of  a  film  of  adsorbed 
gas  in  an  apparently  liquefied  state  stands  in  no  relation  to  the 
vapor  pressure  of  a  mass  of  the  liquefied  gas.  The  phase  rule 
generalization  that  the  vapor  pressure  is  constant  for  the  system, 
liquid  and  vapor  at  constant  temperature,  holds  good  explicitly 
only  in  case  the  disturbing  effects  due  to  gravity,  distortion  of 
the  solid  masses,  and  capillary  tensions  are  eliminated.  So 
soon  as  a  liquid  film  becomes  thin  enough  so  that  the  liquid  no 
longer  has  the  properties  of  matter  in  mass,  the  vapor  pressure 
need  not  remain  constant.  When  we  have  an  adsorbed  liquid 
or  liquefied  gas,  the  vapor  pressure  will  vary  also  with  the  nature 
of  the  adsorbing  agent.  The  lowering  of  the  vapor  pressure  is 
shown  by  the  fact  that  oatmeal,1  previously  dried  at  high  tem- 
perature, has  been  used  instead  of  sulphuric  acid  to  produce  the 
•  freezing  of  water  under  the  receiver  of  an  air-pump.  L.  J.  Briggs2 
showed  that  the  weight  of  the  water  film  condensed  on  the  sur- 
face of  quartz  at  30°  in  an  atmosphere  within  one  percent  of 
saturation  is  26.6  mg  per  square  meter  as  determined  by  drying 
afc  110°.  This  corresponds  to  a  calculated  thickness  of  the  water 
film  of  26.6juju  assuming  the  density  of  the  film  to  be  uni- 
form and  equal  to  unity.  Neither  assumption  is  absolutely 
accurate. 

Since  the  amount  of  adsorption  increases  rapidly  with  falling 
temperature,  adsorption  must  be  accompanied  by  a  marked 
evolution  of  heat.  In  Table  XI  the  data  not  bracketted  are  by 
Favre.3  All  the  molecular  heats  of  adsorption  are  higher  than 
the  corresponding  heats  of  liquefaction.  The  abnormally  high 
values  for  hydrogen  in  palladium  and  platinum  cannot  be  due  to 
dissociation  into  monatomic  hydrogen,  because  that  would  be 
accompanied  by  an  adsorption  of  heat.  They  may  be  due  in 
part  to  an  oxidation  of  hydrogen.  Masson4  considers  that  the 
heat  of  adsorption  of  water  vapor  does  not  differ  materially 
from  the  heat  of  liquefaction;  but  Dewar's  data,5  Table  XII, 
confirm  those  of  Favre,  and  we  know  also  that  there  is  always  a 

1  THOMSON:  Phil.  Mag.  (4)  42,  448  (1871). 

2  Jour.  Phys.  Chem.,  9,  617  (1905). 

3  Ann.  Chim.  Phys.  (5)  1,  209  (1874). 

4  Proc.  Roy.  Soc.,  74,  249  (1904). 
SDEWAR:  Proc.  Roy.  Inst.,  18,  183  (1905). 


26 


APPLIED  COLLOID  CHEMISTRY 


heat  effect  when  porous  solids  are  wetted  by  liquids.1  This  is 
probably  due  to  an  actual  increase  in  the  density  of  the  adsorbed 
liquid. 

TABLE  XI. — MOLECULAR  HEATS  OF  ADSORPTION  AND  OP   LIQUEFACTION 

OF  GASES 


Adsorbing  agent 

Gas 

Adsorption 
g  cal.  per  mol. 

Liquefaction 
g  cal.  per  mol. 

Platinum          

H2 

46,200 

(240) 

Palladium                

H2 

18,000 

(240) 

Charcoal                                      •  • 

NH3 

5?900  —  8,500 

(5,000) 

Charcoal 

CO2 

6,800  —  7,800 

6,250 

Charcoal                

N2O 

7,100—  7,700 

4,400 

Charcoal                      .  .          ... 

SO2 

10,000—10,900 

5,600 

Charcoal 

HC1 

9,200  —  10,200 

(3,600) 

Charcoal.               

HBr 

15,200—15,800 

(4,000) 

Charcoal                                .      .  . 

HI 

21,000  —  23,000 

(4,400) 

Melsens2  found  that  when  25  cc  of  different  liquids  were 
added  to  10  grams  of  freshly  ignited  wood  charcoal  there  was  a 
rise  of  temperature  of  several  degrees  with  ether,  alcohol,  and 

TABLE  XII. — MOLECULAR  HEATS  OF  LIQUEFACTION  AND  ADSORPTION  OF 

GASES 

Charcoal  as  adsorbing  agent  at  — 185°  (Dewar). 


Gas 

Adsorption 
g.  cal.  per  mol. 

Liquefaction, 
g.  cal.  per  mol. 

Hvdroffen 

1600 

238 

Nitrogen                               

3686 

1372 

Argon 

3636 

Oxvsren 

3744 

1664 

Carbon  monoxide             

3416 

carbon  bisulphide.  When  97  grams  of  bromine  were  added  to 
11  grams  of  charcoal  the  rise  of  temperature  was  about  30°. 
Since  a  sudden  compression  of  ten  atmospheres  raises  the  tem- 

^f.  POUILLET:  Ann.  Chim.  Phys.  (2)  20,  141  (1822);  CHAPPUIS:  Wied. 
Ann.  19,  21  (1883);  MARTINE:  Phil.  Mag.  (5)  47,  329  (1889);  60,  618  (1900); 
(6)  15,  595  (1903);  PARKS:  Ibid.  (6)  4,  240  (1902);  6,  517  (1903). 

2  Ann.  Chim.  Phys.  (5)  3,  522  (1874). 


ADSORPTION  OF  GAS  OR  VAPOR  BY  SOLID 


27 


perature  of  water  only  1/77  of  a  degree  he  concluded  that  the 
rise  of  temperature  of  1.16°  when  charcoal  is  wetted  by  water 
corresponds  to  a  pressure  of  893  atmospheres.  Calculating  in 
the  same  way  he  drew  the  conclusion  that  the  force  of  adsorption 
is  3100  atmospheres  for  bromine,  13,090  for  carbon  bisulphide, 
4620  for  ether  and  3080  for  alcohol. 

Gaudechon1  obtained  the  data  given  in  Table  XIII  for  different 
liquids,  with  wood  charcoal.  Since  the  heat  of  liquefaction  varies 
with  the  temperature,  it  is  probable  that  the  heat  of  adsorption  of 
gases  and  vapors  does  also;  but  this  point  seems  not  to  have 
received  any  special  attention. 

TABLE  XIII. — HEAT  EVOLUTION   ON  WETTING   WOOD   CHARCOAL 
Gram  calories  per  gram  charcoal. 


g.  cal. 

g.  cal. 

Water  

3.9 

Acetone  

3.6 

Methyl  alcohol 

11.5 

Chloroform   . 

2  3 

Ethyl  alcohol 

6  9 

Ether 

1  2 

Propyl  alcohol. 

5.6 

Benzene  

4.2 

Amyl  alcohol 

3  7 

CC14 

1  5 

Formic  acid  

12.0 

CS2  

4.0 

Acetic  acid 

6  0 

0.4 

Lamb ^and  Coolidge2  find  that  curves,  which  approximate 
closely  to  straight  lines,  are  obtained  when  the  heats  of  adsorp- 
tion of  the  vapors  of  some  organic  liquids  by  charcoal  are  plotted 
against  the  amounts  adsorbed.  The  difference  between  the  heat 
of  adsorption  and  the  heat  of  vaporization  is  called  by  them  the 
net  heat  of  adsorption  and  is  practically  constant  for  the  different 
liquids  studied,  when  referred  to  equal  volumes  of  adsorbed 
liquid.  Assuming  that  this  net  heat  of  adsorption  is  a  heat  of 
compression,  the  attracting  force  is  estimated  to  be  about  37,000 
atmospheres  when  one  cubic  centimeter  of  liquid  is  adsorbed  by 
ten  grams  of  charcoal. 

The  action  of  water  vapor  on  cottdn  has  been  studied  by 


1  Comptes  rendus,  157,  209  (1913). 

2  Jour.  Am.  Chem.  Soc.  42,  1146  (1920). 


28  APPLIED  COLLOID  CHEMISTRY 

Beadle1  and  by  Masson.2  If  the  bulb  of  a  thermometer  be 
covered  with  carefully  dried  cotton  wool,  and  if  the  bulb  be  then 
dipped  in  a  beaker  or  water  at  the  same  temperature,  the  thermom- 
eter will  rise  8°-12°  or  even  more.  The  relatively  large  amount 
of  air  which  is  always  entangled  in  the  cotton  wool  does  not 
escape  during  or  after  immersion,  and  the  fibers  present  a  glisten- 
ing appearance  under  water,  which  suggest  that  they  are  not 
wetted  completely.  Medicated  or  absorbent  cotton  differs 
from  the  ordinary,  nearly  pure,  material  in  that  the  former  has 
been  freed  from  traces  of  natural  cotton  wax  by  treatment  with 
solvents.  When  absorbent  cotton  is  dipped  into  water,  the 
rise  in  temperature  is  less  than  in  the  previous  case  even  though 
more  water  is  taken  up  by  the  cotton.  In  the  case  of  the  nearly 
pure  cotton  wool,  the  water  evidently  distills  across  the  air 
space,  while  there  is  a  certain  amount  of  direct  wetting  with  the 
absorbent  cotton.  There  should  therefore  be  a  marked  evolu- 
tion of  heat  if  cotton  is  placed  in  saturated  water  vapor  and  the 
two  cottons  should  behave  exactly  alike.  Both  these  predictions 
were  confirmed  experimentally.  Masson  obtained  similar  re- 
sults when  guncotton  or  glass  wool  was  exposed  to  saturated 
water  vapor  or  cotton  to  the  vapor  of  absolute  alcohol ;  but  the 
heat  effects  were  much  smaller  in  these  instances  owing  to  the 
lesser  adsorption. 

L.  B.  Loeb3  finds  that  the  adsorption  of  methyl  alcohol  vapor 
by  0.41g  platinum  black  causes  a  rise  of  temperature  of  about 
0.36°  when  there  is  no  oxidation. 

The  effect  of  traces  of  impurity  on  the  adsorption  is  shown  in 
some  experiments  by  Berliner.4  His  purified  palladium  foil 
adsorbed  about  860  volumes  of  hydrogen.  If  allowed  to  stand 
in  a  vacuum  for  an  hour,  it  then  adsorbed  practically  no  hydrogen. 
If  left  for  an  hour  or  two  in  the  air,  the  same  thing  happened. 
The  disturbing  factor  is  apparently  a  trace  of  grease,  because 
water  did  not  wet  the  soiled  palladium.  Similar  results  were 
obtained  with  platinum. 

1  Nature,  49,  457  U884);  Chem.  News,  71,  1  (1895);  73,  180  (1896). 

2  Proc.  Roy.  Soc.,  74,  230  (1904);  MASSON    and    RICHARDS:  Ibid.,   78A, 
412  (1906). 

3  Proc.  Nat.  Acad.  Sci.,  6,  107  (1920). 

4  Wied.  Ann.,  36,  803  (1888). 


ADSORPTION  OF  GAS  OR  VAPOR  BY  SOLID 


29 


THE  ADSORPTION  ISOTHERM 

So  far  we  have  only  committed  ourselves  to  the  qualitative 
statement  that  the  amount  of  adsorption  increases  with  increase 
of  pressure.  It  is  now  advisable  to  consider  the  quantitative 
side.  Dewar1  has  made  some  measurements  with  hydrogen 
at  varying  pressures  in  charcoal  at  —185°.  The  results  are 
given  in  Table  XIV.  Dewar  says  in  regard  to  them  that  the 
amount  adsorbed  is  seen  to  increase  with  the  pressure  to  10  atmos- 
pheres, after  which  the  adsorption  in  the  pores  of  the  charcoal 
seems  to  be  independent  of  the  pressure.  At  the  temperature  of 
liquid  air  this  sample  of  charcoal  (6.7  g)  would  not  adsorb  more 
than  about  one  liter  of  hydrogen  even  when  the  pressure  was 
raised  from  10  to  25  atmospheres.  It  appears  that  Dewar 
considers  the  decrease  from  156  cc  to  138  cc  as  due  to  experi- 
mental error  for  he  says  distinctly  that  the  adsorption  had  come 
to  a  limit.  Since  that  time  a  number  of  careful  investigations 
have  been  made  on  the  quantitative  variation  of  the  adsorption 
with  the  pressure.2 

TABLE  XIV. — HYDROGEN  ADSORBED  BY  CHARCOAL  AT  —185° 


Atm. 

cc/gram 

Atm. 

cc/gram 

1 

92 

15 

149 

5 

138 

20 

145 

10 

156 

25 

138 

If  we  plot  the  amounts  of  adsorbed  gas  against  the  pressures, 
we  get  the  so-called  adsorption  isotherm,  a  smooth  curve  concave 
to  the  pressure  axis.  Trouton3  obtained  a  curve  with  the  oppo- 
site flexure  for  the  adsorption  of  water  by  flannel;  but  Masson4 

1  Proc.  Roy.  Inst.,  18,  437  (1906). 

2  TRAVERS:  Proc.  Roy.  Soc.,  78A,  9  (1906);  DAVIS:  Jour.  Chem.  Soc.,  91, 
1066  (1907);  GEDDES:  Drude's  Ann.,  29,  797,  (1907);  BOYLE:  Jour.  Phys. 
Chem.,   12,  484   (1908);  Phil.   Mag.   (6)   17,  377   (1909);    McBAiN:  Ibid 
18,  916  (1909);  Zeit.  phys.  Chem.,  68,  471  (1909);  HOMPRAY:  Ibid.,  74, 
139,  687  (1910);  TITOFP:  Ibid.,  74,  641  (1910);  PIUTTI  AND  MAGLIA:  Gazz. 
chim.  ital.,  40  I,  569  (1910). 

3  Proc.  Roy.  Soc.,  77A,  292  (1905). 

4  Ibid.,  78A,  9  (1906). 


30 


APPLIED  COLLOID  CHEMISTRY 


obtained  normal  curves  for  water  and  cotton,  and  Travers1 
obtained  normal  curves  for  wool  and  cotton  with  water  vapor, 
so  that  Trouton's  results  are  apparently  due  to  experimental 


-7$ 


0  I  A         Z  5  4 

c.c-  J-fe  ADSORBED  F>ER  GRAM  CHARCOAL 

FIG.  1. 

error.  Trouton2  also  obtained  an  abnormal  type  of  curve  for  the 
adsorption  of  water  vapor  by  glass;  but  it  seems  probable  that 
alkali  dissolved  from  the  glass  has  been  a  disturbing  factor. 

1  Proc.  Roy.  Soc.,  79A,  204  (1906). 

2  Ibid.,  79A,  383  (1906). 


ADSORPTION  OF  GAS  OR  VAPOR  BY  SOLID 


31 


If  the  gas  is  adsorbed  but  slightly,  the  curve  may  seem  practi- 
cally a  straight  line  over  the  ordinary  pressure  ranges.  This  is 
shown  very  clearly  in  the  curves  for  hydrogen  in  charcoal  Fig.  1 
as  determined  by  Tit  off.1  If  the  gas  is  adsorbed  strongly,  the 
isotherm  bends  around  so  as  to  run  nearly  parallel  with  the 
pressure  axis,  because  then  a  marked  increase  in  pressure  causes 


FIG.  2. 

only  a  relatively  slight  increase  in  adsorption.  These  two  ex- 
tremes can  be  realized  very  well  with  ammonia  and  charcoal2 
at  175°  and  -64°,  Fig.  2.  In  Table  XV  are  given  Titoff's  data 
for  the  adsorption  of  ammonia  by  charcoal  at  0°  and  at  151.5°. 
Under  Ax/Ay  is  given  the  mean  increase  in  adsorption  with 
increase  of  pressure  between  each  two  consecutive  observations. 

1  Zeit.  phys.  Chem.,  74,  641  (1910). 

2  RICHARDSON:  Jour.  Am.  Chem.  Soc.,  38,  1828  (1917). 


32 


APPLIED  COLLOID  CHEMISTRY 


At  0°  the  values  drop  from  92  to  3,  a  ratio  of  over  30:1,  while  at 
151.5°,  the  drop  is  only  from  51  to  11,  a  ratio  of  less  than  5:1. 
If  we  plot  the  logarithms  of  the  amounts  of  adsorbed  gas 

TABLE  XV. — ADSORPTION  OF  AMMONIA  BY  CHARCOAL 


Temperature  0° 

Temperature  151.5° 

Pressure 

Adsorption 

Ax/Ay 

Pressure 

Adsorption 

Ax/Ay 

in  mm 

in  cc 

cc/mm 

in  mm 

in  cc 

cc/mm 

2.9 

5.4 

2.8 

0.3 

28.8 

30.2 

0.92 

16.6 

1.0 

0.051 

78.7 

60.4 

0.65 

64.8 

2.1 

0.021 

161.0 

90.3 

0.36 

176.2 

4.0 

0.018 

319.2 

115.7 

0.16 

285.0 

5.5 

0.013 

490.0 

127.0 

0.07 

432.7 

7.2 

0.012 

636.4 

132.4 

0.04 

576.8 

8.9 

0.012 

746.7 

135.9 

0.03 

721.8 

10.5 

0.011 

against  the  logarithms  of  the  pressures,  we  get  a  curve  which 
approximates  a  straight  line,  especially  if  the  gas  is  not  adsorbed 
too  much.  This  shows  that  the  data  can  be  represented  approxi- 
mately by  an  equation1  of  the  form  (x/m)n  =  kp,  where  x  is  the 

TABLE  XVI. — ADSORPTION  OF  CARBON  DIOXIDE  BY  CHARCOAL 

Formula  x  ^"/P  =  0.0602. 

x  =  cc  CC>2  adsorbed  per  cc  charcoal. 

p  =  pressure  in  mm  Hg.  Temp.  31°. 


p 

x  found 

x  calc. 

P 

x  found 

x  calc. 

41.5 

1.7 

1.7 

453 

6.5 

6.4 

120. 

2.9 

3.1 

534 

7.1 

7.1 

194. 

4.0 

4.0 

602 

7.6 

7.6 

276. 

4.7 

4.9 

678 

8.3 

8.2 

340. 

5.4 

5.5 

698 

8.6 

8.3 

405. 

5.9 

6.0 

703 

8.9 

8.4 

amount  of  gas  adsorbed  and  m  the  amount  of  the  adsorbing 
solid,  while  p  is  the  pressure  and  k  and  n  are  constants  to  be 
determined  experimentally  for  each  temperature.  The  expo- 


1  Chemists  are  in  the  habit  of  calling  this  an  exponential  formula  which  is 
not  correct  in  the  strict  mathematical  sense;  but  which  is  justified  by  usage. 


ADSORPTION  OF  GAS  OR  VAPOR  BY  SOLID 


33 


nential  factor,  n,  is  usually  not  an  integer  and  its  physical  signifi- 
cance is  unknown.1  In  Table  XVI  are  given  some  data  by 
Geddes2  on  the  adsorption  of  C02  by  charcoal.  The  formula 
enables  us  to  calculate  the  adsorption  with  a  moderate  degree  of 
accuracy.  Of  course  data  could  have  been  picked  out  which 
would  have  looked  better,  but  there  seems  no  advantage  in 
doing  this.  The  formula  cannot  possibly  represent  the  facts 
for  pressures  at  which  the  curve  runs  nearly  parallel  to  the  pres- 
sure axis,  and  the  experiments  of  Richardson3  on  the  adsorption 


FIG.  3. 

of  ammonia  by  charcoal  makes  it  probable  that  the  equation 
never  describes  the  facts  accurately  over  any  wide  range  of 
pressures.  For  the  present  it  should  be  considered  as  a  service- 
able interpolation  formula,  though  it  may  possibly  belong  in  the 
same  category  as  the  simple  gas  law. 

The  fact  that  we  get  a  smooth  curve  when  plotting  pressure 
direct  against  adsorption  shows  that  no  definite  compound  is 
formed.  In  Fig.  3  the  pressure  is  plotted  along  the  vertical 
axis  and  the  amount  of  gas  condensed  on  or  by  unit  mass  is 

1  For  a  slightly  different  formulation,  see  WILLIAMS:  Trans.  Faraday  Soc., 
10,  167  (1914);  Proc.  Roy.  Soe.,  Edinburgh,  391,  48  (1919). 

2  Drude's  Ann.,  29,  797  (1909). 

3  Jour.  Am.  Chem.  Soc.,  38,  1828  (1917). 

3 


34  APPLIED  COLLOID  CHEMISTRY 

plotted  along  the  horizontal  axis.  The  typical  adsorption  iso- 
therm is  represented  by  the  curve  OH,  while  OABCDE  is  the 
curve  (not  to  scale)  for  the  taking  up  of  water  by  sodium  sul- 
phate. Along  OA  the  solid  phase  is  anhydrous  sodium  sul- 
phate. Along  AB  the  pressure  remains  constant  because  three 
phases  are  present:  Na2SO4,  Na2SO4.10H20,  and  water  vapor. 
Along  BC  the  pressures  change  without  any  more  water  being 
taken  up  and  consequently  there  are  only  two  phases,  water 
vapor  and  the  compound  Na2S04.10H2O.  The  composition 
of  the  compound  is  given  by  F,  the  point  at  which  the  prolonga- 
tion of  CB  cuts  the  axis  of  abscissas.  Along  CD  the  phases  are 
Na2SO4.10H2O,  saturated  solution  and  vapor,  so  the  pressure 
remains  constant,  while  unsaturated  solution  and  vapor  coexist 
along  the  curve  DE.  Many  curves  of  this  type  have  been  de- 
termined by  Browne1  and  others  in  his  laboratory  for  the  am- 
monia compounds.  The  curve  OH  is  clearly  not  of  this  type  and 
represents  either  a  continuous  series  of  solid  solutions  or  an  ad- 
sorption. The  equilibrium  is  usually  reached  fairly  rapidly, 
which  is  an  argument  against  the  existence  of  a  solid  solution 
though  by  no  means  a  conclusive  one.  More  convincing  is  the 
fact  that  the  amount  of  adsorption  by  charcoal  varies  with  the 
structure2  and  the  extent  of  surface.  The  change  produced  by 
grinding  charcoal3  is  only  a  slight  one,  because  the  surface  of  the 
unground  charcoal  is  so  enormous.  Lagergren4  estimates  that 
one  gram  of  charcoal  has  a  surface  of  four  square  meters  and  is 
composed  of  about  1.4  billion  particles.  Lamb,  Wilson  and 
Chaney5  consider  that  one  cubic  centimeter  of  active  war  charcoal 
has  a  surface  of  about  1000  square  meters  (density  about  0.4). 

While  the  experiments  on  the  taking  up  of  gases  by  charcoal 
show  fairly  conclusively  that  surface  phenomena  are  the  im- 
portant ones,  there  is  always  the  possibility  that  a  solid  solution 
may  also  be  formed  to  some  extent.  McBain6  considers  that 

1  See  BROWNE  and  HOULEHAN:  Jour.  Am.  Chem.  Soc.,  35,  649  (1913). 

2  PIUTTI  and  MAGLIA:  Gazz.  chim.  Hal.,  40  I,  569  (1910). 
3CHAPPUis:  Wied.  Ann.,   12,   164   (1881).     Cf.  also  GURWITSCH:  Zeit. 

Kolloidchemie,  11,  17  (1912). 

4  See  VAN  BEMMELEN:  Die  Absorption,  410  (1910). 

5  Jour.  Ind.  Eng.  Chem.,  11,  420  (1919). 

6  Phil.  Mag.  (6)  18,  916;  Zeit.  phys.  Chem.,  68,  471  (1909). 


ADSORPTION  OF  GAS  OR  VAPOR  BY  SOLID  35 

over  13  percent  of  the  hydrogen  taken  up  by  cocoanut  charcoal 
at  the  temperature  of  liquid  air  is  present  as  a  solid  solution.1 
On  the  other  hand  Titoff2  believes  that  the  amount  of  hydrogen 
dissolved  in  the  charcoal  has  been  over-estimated  by  McBain.3 
The  truth  of  the  matter  is  that  we  have  no  definite  criterion  which 
will  enable  us  to  distinguish  accurately  in  all  cases  between  solid 
solution  and  adsorption.  In  the  case  of  cellulose  and  of  gelatine 
it  is  quite  possible  that  some  water  is  actually  dissolved.  Until 
we  know  more  about  the  subject  it  is  safer  to  treat  any  doubtful 
case  as  chiefly  one  of  adsorption. 

On  the  other  hand,  the  two  special  cases  of  metals  and  rubber 
with  gases  call  for  some  special  comment.  Assuming  the  exist- 
ence of  pores  in  platinum  or  palladium  foil,  one  can  account  for 
the  selective  diffusion  of  hydrogen  by  postulating  adsorption  in 
the  pores  and  subsequent  diffusion.  At  high  temperatures  the 
adsorption  would  be  much  less,  and  it  is  difficult  to  explain  the 
increased  rate  of  selective  diffusion.  It  is  simpler  to  account  for 
the  diffusion  of  hydrogen  through  platinum  by  postulating  that 
some  of  the  hydrogen  actually  dissolves  in  the  platinum.  In  that 
case  the  solubility  should  be  independent  of  the  physical  struc- 
ture of  the  platinum.  This  seems  to  be  true  within  narrow  limits 
for  platinum  and  within  wide  limits  for  palladium.  It  seems 
probable  that  platinum  can  dissolve  several  volumes  of  hydrogen 
at  ordinary  temperatures,  and  palladium  nearly  eight  hundred 
volumes.4  The  bulk  of  the  hundred  volumes  of  hydrogen  which 
are  taken  up  by  platinum  black5  must  be  adsorbed,  and  the  same 
must  be  true  of  the  nearly  3000  volumes  taken  up  by  colloidal 
platinum.6  This  is  apparently  the  conclusion  reached  by  Holt, 
Edgar,  and  Firth.7  If  platinum  and  palladium  dissolve  hydro- 

1  Cf.  TRAVERS:  Proc.  Roy.  Soc.,  78A,  9  (1906). 

2  Zeit.  ph-ys.  Chem.,  74,  641  (1910). 

3  For  other  discussions,  see  MASSON:  Proc.  Roy.  Soc.,  74,  251  (1904); 
DAVIS:  Jour.   Chem.  Soc.,   91,   1066   (1907);  GEDDES:  Dmde's  Ann.,   29, 
797  (1908);  WOHLER,  PLUDDEMANN  and  WOHLER:  Zeit.  phys.  Chem.,  62, 
670  (1908);  HOMFRAY:  Ibid.,  74,  139,  687  (1910). 

4  Cf.  BOSE:  Zeit.  phys.  Chem.,  34,  708  (1900). 

5  RAMSAY,  MOND  and  SHIELDS:  Zeit.  phys.  Chem.,  19,  40  (1896);  25,  657 
(1898). 

6  PAAL  and  GERUM:  Ber.  deutsch.  chem.  Ges.,  41,  805  (1908). 

7  Zeit.  phys.  Chem.,  82,  513  (1913). 


36 


APPLIED  COLLOID  CHEMISTRY 


gen  to  some  extent,  the  passage  of  carbon  monoxide  through 
heated  iron  cannot  be  attributed  to  adsorption.  At  present  it 
is  not  possible  to  say  whether  an  iron  carbonyl  plays  any  part 
in  the  diffusion  or  not.  In  the  case  of  platinum  and  oxygen 
Ramsay,  Mond,  and  Shields1  consider  that  most  of  the  oxygen 
taken  up  forms  Pt  (OH)2.  They  point  out  that  Thomsen  ob- 
tained an  evolution  of  heat  of  17,000  calories  for  the  reaction 
Pt+O+H2O,  while  they  find  17,600  calories  for  the  so-called 
adsorption  of  sixteen  grams  of  oxygen  by  platinum. 

In  view  of  the  relative  behavior  of  rubber  with  water  and  with 
pyridine,  it  is  hard  to  see  how  the  permeability  can  be  a  question 
primarily  of  adsorption.  1  think  that  we  must  assume  that 
carbon  dioxide,  pyridine,  etc.,  actually  dissolve  in  rubber. 


TABLE  XVII. — ADSORPTION  OF  CARBON   DIOXIDE   BY   BLOOD  CHARCOAL 

Formula  x/p  =  0.00024. 
x  =  millimols  CC>2  per  gram  charcoal, 
p  =  partial  pressure  in  mm  Hg. 
Temperature  20°. 


p 

X 

k 

p 

X 

k 

613.1 

0.1461 

0.00024 

337.7 

0.0855 

0.00025 

611.1 

0.1424 

0.00023 

325.5 

0.0808 

0.00025 

605. 

0.1320 

0.00022 

312.2 

0.0706 

0.00023 

453.1 

0.1014 

0.00022 

229. 

0.0560 

0.00024 

437.2 

0.1086 

0.00025 

221.2 

0.0523 

0.00023 

435.2 

0.1063 

0.00024 

176. 

0.0452 

0.00026 

348.5 

0.0844 

0.00024 

170.2 

0.0451 

0.00026 

339. 

0.088 

0.00026 

mean 

0.00024 

While  the  equation  x/m  =  kp  is  a  special  case  of  the  equation 
(x/m)n  =  kp,  where  n  =  1,  and  therefore  may  hold  even  in 
cases  of  adsorption,  it  is  also  the  simplest  case  of  Henry's  law  for 
the  solubility  of  gases  in  liquids.  Reychler2  found  that  the 
amount  of  carbon  dioxide  taken  up  by  blood  charcoal  was  pro- 
portional to  the  partial  pressure  of  the  carbon  dioxide  in  the 
vapor  phase.  The  data  are  given  in  Table  XVII.  It  does  not 

*  Ibid.,  26,  684  (1898).. 

2  Van  Bemmelen  Gedenkboek,  55  (1910). 


ADSORPTION  OF  GAS  OR  VAPOR  BY  SOLID  37 

seem  probable  that  C02  dissolves  in  carbon,  and  I  prefer  to 
consider  this  a  case  of  adsorption  where  the  exponential  factor 
happens  to  be  unity.  This  view  is  confirmed  by  the  fact  that 
Geddes  obtained  very  different  relations  for  the  adsorption  of 
CO2  by  charcoal  at  31°  (Table  XVI). 

Reychler  also  found  that  the  amount  of  carbon  dioxide  taken 
up  by  rubber  was  proportional  to  the  pressure  over  a  narrow 
range  of  pressures.  This  may  well  be  'a  case  of  solution  of 
carbon  dioxide  in  rubber. 

e  concentration-pressure  curve  for  hydrogen  and  palla- 
dium1 has  two  ascending  branches  connected  by  a  branch  which 
is  nearly  but  not  quite,  parallel  to  the  concentration  axis.  It  is 
not  known  to  what  extent  the  peculiarities  of  this  curve  are  due  to 
adsorption. 

While  many  people  are  willing  to  speak  of  adsorption  and  to 
let  it  go  at  that,  attempts  have  been  made  to  account  for  adsorp- 
tion either  through  the  formation  of  known  or  unknown  com- 
pounds.    Rhead  and   Wheeler2  consider  that  the   fixation  of 
oxygen  by  carbon  involves  the  formation  of  a  compound,  CxOy, 
which  breaks  up  into  carbon  dioxide  and  carbon  monoxide  when 
heated.     Haber3  points  out  that,  according  to  Bragg,  the  ordi- 
nary solid  crystalline  salts  are  not  systems  in  which  one  anion 
and  one  cation  form  a  molecule  which  is  separated  from  the  next; 
/  but  that  one  anion  is  probably  bound  to  all  the  surrounding 
\  cations.     Unless  the  outer  layer  of  a  crystal  differs  fundamentally 
/  from  the  inner  portions,  there  must  be  forces  at  the  surface  of  the 
}  crystal  similar  to  those  acting  between  the  outer  layer  and  the 
next  layer.     If  we  call  these  forces  chemical  valences  of  some 
sort,  it  is  possible  that  adsorption  is  due  essentially  to  the  satura- 
tion of  these  valences.     These  ideas  have  been  developed  by 
Langmuir4  as  a  result  of  his  experiments  on  electrically  heated 
carbon  and  other  filaments  in  presence  of  oxygen  at  pressures 
of  0.004-0.007  mm.    Langmuir  considers  that  any  crystal  or 

(  IHOITSEMA:  Zeit.  phys.  Chem.,  17,  1  (1895);  RAMSAY,  MOND,  and 
SHIELDS:  Ibid.,  19,  25  (1896);  HOLT,  EDGAR  and  FIRTH:  Jour.  Chem.  Soc., 
82,  513  (1913). 

2  Jour.  Chem.  Soc.,  103,  462  (1913). 

3  Jour.  Soc.  Chem.  Ind.,  33,  50;  Zeit.  Elektrochemie,  20,  521  (1914). 
« Jour.  Am.  Chem.  Soc.,  37,  1139  (1915);  38,  1145,  2221  (1916). 


38  APPLIED  COLLOID  CHEMISTRY 

any  mass  of  liquid  forms  a  giant  molecule,  and  that  an  adsorbed 
gas  forms  a  compound  with  the  outer  layer,  the  thickness  of  the 
reacting  gas  film  being  rarely  over  one  or  two  molecules  thick. 
Looking  at  the  subject  from  a  kinetic  point  of  view  he  deduces 
an  equation  of  the  form 


where  Q  is  the  amount  adsorbed  at  the  pressure  p,  and  a,  b 
and  c  are  constants. 

There  are  two  independent  questions  involved  in  Langmuir's 
hypothesis,  namely  whether  the  maximum  adsorption  film  is 
only  one  or  two  molecules  thick  and  whether  this  layer  forms  a 
compound  with  the  adsorbing  agent.  The  first  may  be  true 
and  the  second  be  wrong.  Langmuir  claims  that  the  adsorbed 
film  never  exceeds  two  molecules  in  thickness  for  plane  surfaces, 
though  he  is  apparently  willing  to  concede  a  greater  thickness 
for  porous  substances.  There  is  no  question  but  that  the  adsorp- 
tion film  is  very  thin.  Mitscherlich1  calculates  a  thickness  of 
about  50fjLfj,  for  carbon  dioxide  adsorbed  by  charcoal,  and  the 
film  of  water  round  each  particle  of  clay2  is  usually  assumed  to  be 
of  the  same  thickness,  while  L.  J.  Briggs3  estimates  about  27/^c 
for  the  film  of  water  on  quartz.  It  is  difficult  to  be  certain  on 
these  points  because  one  does  not  know  the  surface  with  any 
degree  of  accuracy;  but  the  evidence  is  in  favor  of  Langmuir's 
contention. 

When  it  comes  to  the  question  of  whether  compounds  are 
formed  or  not,  it  must  be  remembered  that  Langmuir  uses  the 
word  molecule  in  a  new  sense.  He  considers  that  the  oceans  and 
all  that  in  them  is  constitute  one  vast  molecule  and  that  catch- 
ing a  fish  is  a  chemical  reaction  because  it  breaks  up  the  molecule. 
This  may  be  perfectly  true;  but  the  usefulness  of  it  has  not  been 
made  clear.  For  the  present,  it  seems  much  wiser  to  say  that 
one  substance  adsorbs  another  selectively,  meaning  that  the 
concentration  at  the  surface  of  the  phase  has  increased.  When- 
ever there  is  no  proof  that  a  compound  is  formed  of  the  type  at 

1  SITZUNGSBER-.  Akad.  Wiss.  Berlin,  1841,  379. 

2  Cf.  DAVIS:  Trans.  Am.  Ceramic  Soc.,  16,  69  (1913), 

3  Jour.  Phys.  Chem.,  9,  617  (1905), 


ADSORPTION  OF  GAS  OR  VAPOR  BY  SOLID  39 

present  recognized  by  chemists,  it  is  safer  not  to  postulate  the 
existence  of  compounds.  When  the  time  comes  that  it  is  a  dis- 
tinct advantage  to  distinguish  an  entirely  new  type  of  compound, 
an  indefinite  chemical  compound,  that  can  be  done;  but  hopeless 
confusion  will  arise  whenever  a  man  uses  a  word  in  one  sense  and 
his  listeners  in  another.  It  is  quite  probable  that  a  more  careful 
study  of  contact  catalysis  may  force  us  to  adopt  Langmuir's 
view  or  some  modification  of  it;  but  it  will  then  be  necessary  to 
gather  in  a  lot  of  the  loose  ends. 

In  some  cases  compounds  are  undoubtedly  formed.  Thus 
when  platinum  black  takes  up  oxygen,  a  portion  of  the  oxygen 
is  certainly  present  as  oxide.  On  the  other  hand,  it  seems  im- 
probable that  it  would  be  so  difficult  to  make  carbon  tetrachloride 
direct  if  the  chlorine,  which  is  adsorbed  by  charcoal  and  there- 
by made  active,1  consisted  of  a  normal  compound  of  carbon  and 
chlorine.  While  the  oxidizing  action  of  platinum  may  be  due  to 
a  platinum  oxide,  this  cannot  be  true  in  the  case  of  charcoal  and 
oxygen.  Calvert2  showed  that  oxygen  adsorbed  by  charcoal 
will  oxidize  ethyl  alcohol  to  acetic  acid  and  ethylene  to  carbon 
dioxide  and  water,  reactions  which  certainly  cannot  be  due  to 
any  known  oxide  of  carbon.  It  also  seems  unsatisfactory  to 
postulate  that  the  noble  gases  form  definite  chemical  compounds 
with  charcoal. 

Years  ago  Angus  Smith3  ignored  the  variation  of  adsorption 
with  the  pressure  and  concluded  that  at  atmospheric  pressure 
charcoal  adsorbed  gases  in  volumes  which  were  multiples  of 
the  volumes  ot  hydrogen  adsorbed.  His  figures  were:  hydrogen, 
1;  oxygen,  7.99;  carbon  monoxide,  6.03;  carbon  dioxide,  22.05; 
methane,  10.01;  nitrous  oxide,  12.90;  sulphur  dioxide,  36.95; 
nitrogen,  4.66.  Smith  lays  stress  on  the  fact  that  the  22  for 
carbon  dioxide  and  the  10  for  methane  can  be  considered  as  6 
+ 16  and  6  +  4  respectively,  though  it  is  difficult  to  see  what  this 
proves.  In  order  to  make  the  value  for  nitrogen  seem  plausible, 
Smith  points  out  that  14  X  4.66  =  (14)1;  but  these  calcula- 
tions are  only  of  historical  interest  now. 

1  DAMOISEAU:  Comptes  rendus,  73,  60  (1876). 

2  Jour.  Chem.  Soc.,  20,  293  (1867). 

3Chem.  News,  18,  21  (1868);  Proc.  Roy.  Soc.,  28,  322  (1879). 


40  APPLIED  COLLOID  CHEMISTRY 

CHEMICAL  REACTIONS 

Since  reaction  velocity  is  probably  directly  proportional  to  the 
difference  of  chemical  potential  and  inversely  proportional  to 
the  chemical  resistance,  we  can  increase  the  reaction  velocity 
either  by  increasing  the  difference  of  chemical  potential  or  by 
decreasing  the  chemical  resistance.  The  increased  concentra- 
tion due  to  adsorption  at  the  surface  of  a  solid  will  necessarily 
cause  an  increase  in  reaction  velocity  quite  apart  from  any 
catalytic  action  which  the  solid  may  exert.1  In  the  form  of 
wood  ashes,  charcoal  is  often  used  in  outhouses  and  elsewhere 
to  eliminate  unpleasant  odors.  The  charcoal  adsorbs  the  odori- 
ferous substances  and  then  accelerates  their  oxidation  by  oxygen 
which  is  adsorbed  from  the  air.  Shenstone2  invented  a  respira- 
tor filled  with  powdered  wood  charcoal  to  be  worn  in  hospitals 
and  which  should  adsorb  and  then  destroy  germs  of  cholera  and 
other  diseases.  This  was  the  forerunner  of  the  modern  gas 
mask.  It  is  not  known  whether  the  oxidizing  action  of  the  char- 
coal is  due  to  anything  more  than  the  increase  in  concentration 
of  the  oxygen. 

As  a  matter  of  fact,  the  effect  of  concentration  seems  to  be 
relatively  unimportant  in  most  of  the  cases  studied  hitherto. 
Patrick's  silica  gel  has  a  surface  of  2.5  X  108  cm2  per  gram  and  is 
an  excellent  adsorbent,  but,  so  far,  it  does  not  have  any  marked 
catalytic  effect  except  in  the  oxidation  of  nitric  oxide.  It  would 
be  interesting  to  try  this  substance  at  100°  with  the  mixed  vapors 
of  alcohol  and  acetic  acid,  because  this  reaction  takes  place  at  a 
measurable  rate  in  the  vapor  phase  in  the  absence  of  any  catalytic 
agent.  It  is  stated  that  oxyhydrogen  gas  has  been  found  to  be 
quite  stable  under  pressures  of  two  thousand  atmospheres,  which 
shows  that  the  catalytic  action  of  platinum  on  mixtures  of  hydro- 
gen and  oxygen  must  be  due  to  something  more  than  a  change 
in  concentration.  The  marked  difference  in  the  behavior  of 
platinum  and  charcoal  toward  oxyhydrogen  gas  is  another  argu- 
ment in  favor  of  the  action  of  a  catalytic  agent  being  specific. 
On  the  other  hand  Hempel3  with  S02  and  02  obtained  a  yield 

1  See,  however,  LANGMUIR:  Jour.  Am.  Chem.  Soc.,  38,  2286  (1916). 

2  Pharm.  Jour.,  13,  454  (1854). 

3  Ber.  deutsch.  chem.  Ges.,  23,  445  (1890). 


ADSORPTION  OF  GAS  OR  VAPOR  BY  SOLID  41 

of  65  percent  S03  without  any  catalytic  agent  by  working  at  a 
pressure  of  thirty  to  forty  atmospheres. 

If  we  have  selective  adsorption  of  the  reaction  products,  this 
will  cause  a  lowering  of  the  chemical  potential  of  the  resultants1 
and  will  therefore  increase  the  reaction  velocity.  It  may  even 
determine  the  direction  of  the  reaction.  Alcohol  breaks  down  into 
acetaldehyde  and  hydrogen  in  presence  of  pulverulent  nickel  and 
into  ethylene  and  water  in  presence  of  pulverulent  alumina  or 
silica.2  Since  nickel  adsorbs  hydrogen  strongly  and  alumina 
adsorbs  water,3  it  seems  natural  to  assume  that  the  selective 
adsorption  is  the  determining  factor.4 

While  this  seems  very  satisfactory,  there  are  certain  points 
which  must  not  be  overlooked.  When  making  ethylene  at  Edge- 
wood  Arsenal  during  the  war,  it  was  found  advisable  to  have  a 
large  amount  of  steam  present  with  the  alcohol  vapor  in  order  to 
make  temperature  regulation  easier.  This  undoubtedly  decreased 
the  rate  of  decomposition  of  the  alcohol;  but  that  difficulty  was 
overcome  by  working  at  a  higher  temperature.  It  is  very  diffi- 
cult to  see  how  alumina  can  dehydrate  alcohol  in  presence  of  a 
large  amount  of  water  vapor  if  the  reason  the  alumina  acts  is  be- 
cause of  its  strong  adsorption  of  water  vapor.  Since  there  seems 
very  little  probability  of  accounting  for  the  various  decomposi- 
tions of  the  esters  on  the  basis  of  selective  adsorption  of  the  re- 
action products,  it  is  probable  that  a  complete  explanation  of  the 
phenomena  of  contact  catalysis  is  not  to  be  found  along  these 
lines. 

If  changes  in  the  chemical  potential  are  not  the  important 
factors  in  contact  catalysis,  the  catalytic  agent  must  change  the 
chemical  resistance  in  some  way,  and  there  are  two  possible 
ways  in  which  this  might  happen.  We  may  have  more  effective 
collisions  between  existing  molecules  or  we  may  have  a  conversion 
of  one  or  more  of  the  reacting  substances  into  an  active  form. 

According  to  the  kinetic  theory  the  reaction  velocity  is  pro- 

1  W.  C.  McC.  LEWIS  suggests  the  terms  reactants  and  resultants  for  react- 
ing substances  and  reaction  products. 

2  IPATIEFF:  Ber. deutsch. chem.  Ges.,  34,3579  (1901);  35,  1047,  1057,  (1902) 
36,   1990,   2003,   2014,   2016   (1903);  SABATIER  and  SENDERENS:  Comptes 
rendus,  136,  738,  921,  936,  983  (1903). 

3  JOHNSON:  Jour.  Am.  Chem.  Soc.,  34,  911  (1912). 

4  ENGELDER:  Jour.  Phys.  Chem.,  21,  676  (1917). 


42  APPLIED  COLLOID  CHEMISTRY 

portional  to  the  number  of  collisions  between  possibly  reacting 
molecules;  but  it  does  not  follow  at  all  that  two  molecules  react 
every. time  they  collide.  If  a  large  number  of  collisions  is  neces- 
sary on  an  average  before  a  pair  of  molecules  react,  anything 
which  would  make  these  collisions  more  helpful  might  increase 
the  reaction  velocity  enormously.  The  first  question  is  then 
whether  there  is  any  evidence  of  ineffective  collisions.  This 
matter  has  been  studied  by  Strutt1  who  comes  to  the  conclusion 
that  a  molecule  of  ozone  reacts  every  time  it  strikes  a  molecule 
of  silver  oxide;  but  that  a  molecule  of  active  nitrogen  collides 
with  a  molecule  of  copper  oxide  five  hundred  times  on  an  average 
before  they  react,  while  two  molecules  of  ozone  at  100°  collide 
on  an  average  6  X  10 u  times  before  they  react.  Without  insist- 
ing on  the  absolute  accuracy  of  these  figures  there  is  evidently 
plenty  of  margin  for  an  increase  in  reaction  velocity  with  ozone  at 
100°  if  one  could  produce  more  effective  collisions.  Langmuir2 
finds  that  15  percent  of  all  oxygen  molecules  at  a  pressure  of 
not  over  5  bars  striking  a  tungsten  filament  at  1770°K  react 
with  it  to  form  tungstic  oxide,  WO3.  This  coefficient  increases 
at  higher  temperatures  and  at  3300°K  about  50  percent  of  all 
the  oxygen  molecules  which  strike  the  filament  react  with  it  to 
form  WO 3.  Since  there  are  three  atoms  of  oxygen  in  WO3  and 
only  two  in  the  oxygen  molecule,  Langmuir  considers  that  at 
least  one-half  of  the  tungsten  surface  must  be  covered  at  3300°K 
with  oxygen  in  some  form. 

Tt  is  possible  that  a  catalytic  agent  may  cause  one  molecule 
to  strike  another  amidships  instead  of  head-on  and  may  thereby 
increase  the  effectiveness  of  the  collisions.  It  is  not  impossible 
that  part  at  least  of  the  effect  of  solvents  on  reaction  velocity 
may  be  due  to  some  such  thing  as  this.  If  we  adopt  Langmuir's 
views3  of  oriented  adsorption,  all  sorts  of  things  become  possible. 
If  ethyl  acetate,  for  instance,  attaches  itself  to  one  adsorbent 
by  the  methyl  group,  to  another  by  the  ethyl  group,  and  to  a 
third  by  the  carboxyl  group,  it  might  very  well  be  that  bombard- 
ment of  the  captive  molecule  by  free  ones  might  lead  to  very 
different  reaction  products  in  the  three  cases.  Such  a  suggestion 

1  Proc.  Roy.  Soc.,  87A,  302  (1912). 

2  Jour.  Am.  Chem.  Soc.,  35,  105  (1913);  38,  2270  (1916). 

3  Cf.  DEBUS:  Jour.  Chem.  Soc.,  53,  327  (1888). 


ADSORPTION  OF  GAS  OR  VAPOR  BY  SOLID  43 

is  of  very  little  value,  however,  unless  it  can  be  made  definite. 
We  do  not  know  as  yet  whether  ethyl  acetate  is  actually  adsorbed 
in  one  way  by  nickel,  in  another  way  by  thoria,  and  in  a  third 
way  by  titania,  nor  do  we  know  whether  the  difference  in  the 
manner  of  adsorption,  assuming  it  to  occur,  is  of  such  a  nature 
as  to  account  for  the  differences  in  the  reaction  products.  It  may 
well  be  that  some  entirely  different  hypothesis  will  prove  neces- 
sary. The  general  problem  of  increasing  the  effectiveness  of 
molecular  collisions  is  distinctly  an  important  one  which  has  not 
been  studied  at  all. 

There  is  one  special  case  in  which  we  can  apparently  identify 
one  of  the  factors  of  chemical  resistance.  Buchbock1  studied  the 
decomposition  of  carbonyl  sulphide  and  found  that  the  reaction 
velocity  decreased  with  increasing  viscosity  of  the  solution. 
Since  the  reaction  products  are  gases,  this  suggests  the  possibility 
that  the  retardation  is  due  to  the  greater  difficulty  of  forming 
gas  bubbles  in  a  more  viscous  medium.  This  receives  confirma- 
tion from  Raschig's  work  on  the  preparation  of  hydrazine,2  by 
the  action  of  hypochlorite  on  ammonia,  the  evolution  of  nitrogen 
being  kept  down  by  something  like  starch  to  make  the  solution 
more  viscous.  The  preliminary  reaction  is  NH3  +  NaCIO  = 
NH2C1  +  NaOH  and  high  viscosity  promotes  the  reaction 
NH2C1  +  NH3  =  N2H4HC1  as  against  the  one  3NH2C1  + 
2NH3  =  3NH4C1  +  N2. 

Instead  of  more  effective  collisions  between  unchanged  por- 
tions of  the  reacting  molecules  we  may  have  a  partial  conversion 
of  one  or  more  of  the  reacting  substances  into  active  forms  either 
through  the  rupture  of  normal  valences  or  of  contra-valences, 
opening  up  fields  of  force  as  Baly3  puts  it. 

The  conception  of  Baly  that  a  catalytic  agent  activates  the 
reacting  substances  by  opening  up  fields  of  force  covers  the 
ground  admirably  as  a  purely  formal  statement.  The  trouble 
with  it  is  that  it  is  as  yet  too  vague  to  be  of  any  value  as  a  working 
hypothesis,  though  it  is  undoubtedly  the  best  starting-point. 
It  is  necessary  to  show  in  each  particular  case  just  what  happens 
and  why  it  happens.  While  the  conversion  of  a  reacting  sub- 

1  Zeit.  phys.  Chem.,  23,  123  (1897);  34,  229  (1900). 

2  HENDERSON:  Catalysis  in  Industrial  Chemistry,  60  (1919). 

3  Jour.  Chem.  Soc.,  101,  1469,  1475  (1912). 


44  APPLIED  COLLOID  CHEMISTRY 

stance  into  an  active  form  may  very  well*account  for  the  effect 
of  the  solvent  on  reaction  velocity  in  homogeneous  systems,  it 
must  be  shown  what  active  form  is  produced  and  why  one  sol- 
vent produces  more  of  it  than  another.  Activation  may  lead 
to  the  formation  of  intermediate  compounds  or  it  may  not;  but 
we  have  got  to  get  down  to  more  definiteness  in  all  the  cases  of 
contact  catalysis.  It  is  often  assumed  that  definite  intermediate 
compounds  are  formed  and  this  is  certainly  true  in  some  cases. 
In  the  catalysis  of  hydrogen  peroxide  by  mercury,  the  interme- 
diate formation  of  mercuric  peroxide  can  be  detected  by  the 
eye  because  there  is  an  intermittent  formation  of  a  film,  which 
then  breaks  down,  only  to  form  again.1  The  formation  of 
graphite  is  usually  preceded  by  the  formation  of  a  carbide.  The 
conversion  of  acetic  acid  into  acetone2  by  passing  the  vapor  over 
heated  barium  carbonate  presumably  involves  the  intermediate 
formation  of  barium  acetate,  though  one  cannot  ear-mark  the 
carbon  dioxide  and  so  cannot  be  absolutely  certain  that  the  barium 
carbonate  is  continually  decomposed  and  regenerated.  In  the 
Deacon  chlorine  process,  where  a  mixture  of  hydrochloric  acid 
and  oxygen  is  passed  over  heated  cupric  chloride,  Hurter3 
assumes  the  existence  of  an  intermediate  compound.  People 
are  not  agreed,  however,  whether  the  intermediate  compound  is 
cuprous  chloride,  cupric  oxychloride,  or  both.  The  extreme 
sensitiveness  of  this  reaction  to  poisons  raises  a  question  whether 
an  intermediate  compound  really  is  formed.  In  the  catalytic 
oxidation  of  carbon  monoxide,  it  is  usually  believed  that  there 
is  alternate  reduction  and  oxidation  of  the  oxides  which  act  as 
catalytic  agents. 

Sabatier4  believes  that^  the  catalytic  action  of  alumina  on 
alcohol  is  due  to  the  intermediate  formation  of  an  instable  com- 
pound of  alumina  and  water.  On  the  other  hand,  Henderson5 
assumes  the  intermediate  formation  of  aluminum  ethylate. 
Neither  one  offers  any  experimental  evidence  in  support  of  his 

1  BREDIG  and  VON  ANTROPOFF:  Zeit.  Elektrochemie,  12,  581  (1906);  VON 
ANTROPOFF:  Jour,  prakt.  Chem.  (2)  77,  273  (1908). 

2  SQUIBB:  Jour.  Am.  Chem.  Soc.,  17,  187  (1895). 

3  Jour.  Soc.  Chem.  Ind.,  2,  106  (1883). 

4  Die  Katalyse  in  der  organischen  Chemie,  240  (1914). 
6  Catalysis  in  Industrial  Chemistry,  4  (1919). 


ADSORPTION  OF  GAS  OR  VAPOR  BY  SOLID  45 

belief,  and  neither  one  discusses  why  kaolin  should  act  practi- 
cally as  well  as  alumina. 

It  seems  probable  that  the  best  way  to  solve  the  problem  of 
contact  catalysis  is  to  start  with  the  side  which  has  been  neglected 
a  good  deal  and  to  study  the  selective  action  of  catalytic  agents. 
Quite  apart  from  any  theory  we  may  adopt,  there  are  certain 
questions  which  ought  to  be  answered  experimentally  and  yet 
these  questions'  have  not  even  been  raised  in  a  serious  way.  We 
know  that  ethyl  alcohol  can  be  decomposed  into  acetaldehyde 
and  hydrogen  or  into  ethylene  and  water  but  that  is  practically 
all  that  we  know  as  yet  about  these  reactions.  With  pulverulent 
nickel  the  reaction  is  almost  completely 

CH3CH2OH  =  CH3CH2O  +  H2 

We  do  not  know  whether  the  hydrogen  comes  off  in  one  stage  or 
in  two.  If  it  comes  off  in  two  stages,  is  the  first  stage  CH3CH2O 
or  CH3CHOH?  While  it  may  not  be  easy  to  devise  a  method 
which  will  answer  this  question,  the  problem  is  not  more  difficult 
than  many  which  the  organic  chemists  have  solved  successfully. 
With  pulverulent  alumina  the  reaction  is  almost  completely 

CH3CH2OH  =  CH2CH2  +  H2O. 

Does  water  split  off  as  such  or  do  we  get  the  splitting  off  of 
hydroxyl  or  hydrogen?  It  seems  improbable  that  water  can  split 
off  as  such  because  then  it  would  be  difficult  to  account  for  the 
intermediate  formation  of  ether.  If  the  first  stage  is  a  splitting 
off  of  hydroxyl  does  the  other  hydrogen  come  from  the  adjacent 
carbon  atom  giving  ethylene  direct  or  does  it  come  from  the  same 
carbon  atom,  forming  a  substituted  methylene,  CH3CH,  which 
then  rearranges  to  ethylene?  One  would  also  like  to  know  what 
nickel  does  with  diethyl  ether. 

With  nickel  we  get  the  following  decomposition  of  the  esters: 
CH3C02CH2CH3  =  CH3CH2CH3  +  C02 
CH3CO2CH3  =  CH3CH3  +  CO2 
HC02CH3(?)  =  GH4  +  C02 
With  thoria  the  decomposition  is  quite  different: 
2CH3C02C2H2  =  CH3COCH3  +  C02  +  (C2H5)2O 

=  CH3COCH3  +  CO2  +  C2H6OH  +  C2H4 
2CH3CO2CH3     =  CH3COCH3  +  C02  +  (CH3)2O 
2H  CO2CH3     =  HCHO          +  C02  +  (CH3)2O 


46  APPLIED  COLLOID  CHEMISTRY 

With  titania  we  have  a  third  set  of  products: 
CH3C02C2H5  =  CH3C02H  +  C2H4 
2CH3CO2CH3  =  2CH3C02H  +  C2H4 

HC02CH3  =  HCO2H  +  CH2  =  CO  +  CH3OH 
The  decompositions  are  regular  and  characteristic  with  each 
catalytic  agent  and  the  molecules  must  break  or  slip  at  different 
points  in  the  different  cases;  but  we  certainly  ought  to  know  what 
the  hypothetical  intermediate  stages  are.1  Take  the  case  of 
methyl  acetate  with  nickel,  for  instance.  One  assumption  might 
be  that  we  have  a  methylene  dissociation 

CH3C02CH3  =  CH3C02H  +  CH2  =  CH3CH3+  CO2 
A  second  assumption  is  that  methyl  splits  off: 

CH3C02CH3  =  CH3C02  +  CH3  =  C2H6  +  CO2 
A  third  assumption  might  be  a  splitting  off  of  both  methyl  groups : 

CH3C02CH3  =  2CH3  +  C02  =  C2H6  +  02. 
Is  any  of  these  what  happens  and  how  can  we  prove  it?     Also 
how  does  the  final  assumption  tie  in  with  the  facts  that  in  aqueous 
solution  acetate  ion  goes  to  ethane  and  CO2 

2CH3C02  =  C2H6  +  2C02 

and  that  dry  lead  tetra-acetate  decomposes  at  about  140°  into 
lead  acetate,  methyl  acetate  and  carbon  dioxide 

Pb(CH3C02)4  =  Pb(CH3C02)2  +  CH3CO2CH3  +  C02 
The  question  also  comes  up  as  to  the  changes  in  the  intermediate 
stages  in  case  the  esters  are  chlorinated  more  or  less  completely. 
The  effect  of  the  catalytic  agent  is  very  marked  when  it  comes  to 
the  preferential  oxidation  of  mixed  gases.2  At  low  temperatures, 
the  nature  of  the  catalytic  agent  may  determine  which  of  two 
combustible  gases  will  burn  the  more  rapidly.  In  presence  of 
platinum,  copper  oxide,  or  chamotte,  hydrogen  burns  more 
readily  than  methane,  whereas  the  reverse  is  true  in  borosilicate 
glass  bulbs  at  300°-400°.  Depending  on  conditions,  either 
hydrogen  or  carbon  monoxide  will  burn  the  more  readily.3 

1  This  is  now  being  studied  experimentally  by  PROFESSOR  HOMER  ADKINS. 

2  HENRY:  Phil.    Mag.,   65,   269   (1825);   BONE:  Jour.    Chem.    Soc.,   81, 
535  (1902);  85,  694  (1894);  Proc.  Roy.  Inst.,  19,  82  (1908);  Phil.  Trans. 
215,  298  (1915). 

3  RIDEAL:  Jour.  Chem.  Soc.,  115,  993  (1919). 


ADSORPTION  OF  GAS  OR  VAPOR  BY  SOLID  47 

At  very  high  temperatures  the  specific  effect  of  the  catalytic  agent 
becomes  negligible  because  the  reaction  will  take  place  so  rapidly 
in  the  gas  phase. 

One  very  striking  characteristic  of  solid  catalytic  agents  is 
the  ease  With  which  they  become  inactive  or  are  poisoned.  Since 
the  reaction  takes  place  in  or  at  the  surf  ace,  it  follows  that  any  sub- 
stance which  cuts  down  the  rate  at  which  the  reacting  substances 
reach  the  catalytic  surface1  or  which  prevents  them  from  reach- 
ing it,  will  decrease  the  reaction  velocity  and  may  destroy  the 
catalytic  action  entirely.  Berliner2  has  shown  that  traces  of 
fatty  vapors  from  the  air  or  from  the  grease  on  the  stop-cocks 
will  decrease  the  adsorption  of  hydrogen  by  palladium  from 
nearly  nine  hundred  volumes  practically  to  nothing.  Faraday3 
has  shown  that  traces  of  grease  destroy  the  catalytic  action  of 
platinum  on  oxyhydrogen  gas.  De  Hemptinne4  has  apparently 
shown  that  carbon  monoxide  cuts  down  the  adsorption  of  hydro- 
gen by  palladium,  though  his  method  of  presenting  his  results 
is  very  obscure.  Lunge  and  Harbeck5  found  that  carbon  mon- 
oxide inhibits  practically  completely  the  catalytic  action  of 
platinum  on  a  mixture  of  ethylene  and  hydrogen.  Schonbein6 
pointed  out  that  the  hydrides  of  sulphur,  tellurium,  selenium, 
phosphorus,  arsenic,  and  antimony  act  very  energetically  in 
cutting  down  the  catalytic  action  of  platinum  on  mixtures  of 
air  with  hydrogen  or  ether.  He  considered  that  the  hydride  must 
decompose,  giving  rise  to  a  solid  film.  This  is  not  necessary 
in  order  to  account  for  the  phenomenon;  but  he  seems  to  have 
been  right  in  at  least  one  case,  for  Maxted7  has  found  that  hydro- 
gen sulphide  is  decomposed  by  platinum  black  with  evolution  of 
hydrogen  and  that  the  platinum  then  does  not  adsorb  hydro- 
gen. Langmuir8  believes  that  oxygen  prevents  the  dissociation 
of  hydrogen  by  a  heated  tungsten  filament  because  it  cuts  down 
the  adsorption  of  the  hydrogen. 

1  TAYLOR:  Trans.  Am.  Electrochem.  Soc.,  36  (1919). 

2  Wied.  Ann.,  35,  903  (1888). 

3  Experimental  Researches  on  Electricity,  1,  185  (1839). 

4  Zeit.  phys.  Chem.,  27,  249  (1898). 

5  Zeit.  anorg.  Chem.,  16,  50  (1898). 

6  Jour,  prakt.  Chem.,  29,  238  (1843). 

7  Jour.  Chem.  Soc.,  115,  1050  U919). 

8  Jour.  Am.  Chem.  Soc.,  38,  2272  (1916). 


48  APPLIED  COLLOID  CHEMISTRY 

Harned1  has  shown  that  the  rate  of  adsorption2  of  chloropicrin 
by  a  charcoal  which  has  been  cleaned  by  washing  with  chloropicrin 
is  much  greater  at  first  than  by  a  charcoal  which  has  not  been 
so  cleaned,  although  the  final  equilibrium  is  apparently  about 
the  same  in  the  two  cases.  This  is  analogous  to  the  evaporation 
of  water  when  covered  by  an  oil  film.  The  oil  cuts  down  the 
rate  of  evaporation  very  much  but  has  practically  no  effect  on 
the  partial  pressure  of  water  at  equilibrium.  Taylor  points  out 
that  normally  the  time  of  contact  between  a  gas  and  the  solid 
catalytic  agent  is  extremely  small  and  consequently  anything 
which  decreases  the  rate  of  adsorption  will  cut  down  the  reaction 
velocity  very  much. 

It  is  easy  to  see  that  the  piling  up  of  reaction  products3  will 
cut  down  the  reaction  velocity,  if  they  prevent  the  reacting  sub- 
stances from  coming  in  contact  with  the  catalytic  agent.  This 
has  been  observed  in  the  contact  sulphuric  acid  process.4  The 
explanation  that  the  decrease  in  the  reaction  velocity  is  due 
to  a  decreased  adsorption  of  the  reacting  substances  was  first 
given  by  Fink5  who  is  the  real  pioneer  in  this  line.  Although  the 
reaction  between  carbon  monoxide  and  oxygen  is  practically 
irreversible  at  ordinary  temperature,  Henry6  recognized  that  the 
presence  of  the  reaction  product  might  slow  up  the  rate  of  reaction 
and  he  proved  his  point  by  increasing  the  reaction  velocity  when 
he  removed  the  carbon  dioxide  with  caustic  potash.  Water 
vapor  checks  the  catalytic  dehydration  of  ether7  and  of  alcohol8 
and  hydrogen  cuts  down  the  catalytic  dehydrogenation  of  alcohol. 

When  catalytic  poisons  are  present  or  are  formed  during  the 
reaction,  the  apparent  equilibrium  may  vary  with  the  amount  of 
the  catalytic  agent.  With  only  a  small  amount  present,  the 
catalytic  agent  will  be  poisoned  before  the  reaction  has  run  very 
far.  In  the  hydrolysis  of  ethyl  butyrate  by  enzymes,  the  reaction 

1  Jour.  Am.  Chem.  Soc.,  42,  372  (1020). 

2  TAYLOR:  Trans.  Am.  Electrochem.  Soc.,  36  (1919). 

3  BUNSEN  (1857).    Quoted  by  DEACON:  Jour.  Chem.  Soc.,  23,  736  (1872). 
4 BODLANDER  and  KOPPEN:  Zeit.  Elektrochemie,  9,  566  (1903);  BERL: 

Zeit.  anorg.  Chem.,  44,  267  (1905). 

5  BODENSTEIN  and  FINK:  Zeit.  phys.  Chem.,  60,  61  (1907). 

6  Phil.  Mag.  (3)  9,  324  (1836). 

7  IPATIEFF:  Ber.  deutsch.  chem.  Ges.,  37,  2996  (1904). 
SENGELDER:  Jour.  Phys.  Chem.,  21,  676  (1917). 


ADSORPTION  OF  GAS  OR  VAPOR  BY  SOLID  49 

apparently  runs  to  different  end-points,  depending  on  the  relative 
amounts  of  enzyme.1  One  may  also  get  abnormal  end-points  in 
the  hydrolysis  of  a  solid,  if  the  product  of  hydrolysis  is  also  a 
solid  and  forms  a  film  over  the  reacting  substance.  With  one 
of  the  war  gases,  the  amount  of  hydrochloric  acid  split  off  in 
presence  of  water  was  practically  the  same  when  the  amount  of 
water  added  varied  in  the  ratio  of  one  to  four.  In  other  words, 
an  apparent  equilibrium  was  reached  at  a  given  concentration 
of  hydrochloric  acid  in  one  case  and  at  one-fourth  that  concen- 
tration when  four  times  as  much  water  was  added. 

The  development  of  some  of  the  technical  contact  processes 
has  been  so  interesting  that  it  seems  worth  while  to  discuss  a 
few  of  them  briefly,  beginning  with  the  contact  sulphuric  acid 
process.2 

CONTACT  SULPHURIC  ACID  PROCESS 

In  1831  Phillips  discovered  that  spongy  platinum  increases 
the  rate  at  which  sulphur  dioxide  combines  with  oxygen  according 
to  the  equation  2SO2  +  02  =  2SO3.  He  obtained  a  patent  for 
this  but  could  not  develop  a  commercial  process.  In  1846  Jul- 
lion  patented  the  use  of  platinized  asbestos.  In  1842  Wohler 
and  Mahla  discovered  that  the  oxides  of  copper,  iron,  and  chrom- 
ium act  like  platinum.  In  the  Schroeder-Grillo  contact  material 
hydrated  magnesium  sulphate  is  soaked  in  a  solution  of  a  plati- 
num salt  and  then  heated  in  presence  of  sulphur  dioxide.  A 
thin  layer  of  platinum  is  thus  precipitated  on  the  surface  of  the 
porous,  dehydrated  salt,  0.2-0.3  percent  of  platinum  giving  an 
excellent  catalytic  agent.3 

Knietsch  found  that  a  practically  complete  conversion  of 
sulphur  dioxide  into  sulphur  trioxide  could  be  obtained  when 
sulphur  dioxide  was  diluted  with  air,  the  conversion  being  more 
nearly  complete  the  greater  the  ratio  of  oxygen  to  sulphur 
dioxide. 

When  the  experiments  were  repeated  with  purified  burner  gas 
on  a  commercial  scale,  the  contact  material  soon  became  inert. 
It  was  found  that  this  disastrous  result  was  due  to  impurities 
in  the  burner  gases.  Even  when  present  in  apparently  negligible 

1  KASTLE  and  LOEVENHART:  Am.  Chem.  Jour.,  24,  491  (1900). 

2  KNIETSCH:  Ber.  deutsch.  chem.  Ges.,  34,  4069  (1901). 

3  RIDEAL  and  TAYLOR:  Catalysis  in  Theory  and  Practice,  86  (1919). 


50  APPLIED  COLLOID  CHEMISTRY 

amounts,  these  impurities  play  havoc  with  the  catalytic  action 
of  the  platinum.  Arsenic  is  the  most  dangerous  and  next  mer- 
cury, while  antimony,  bismuth,  lead,  iron,  zinc,  etc.,  act  in  so  far 
as  they  settle  on  and  actually  blanket  the  contact  material. 
An  amount  of  arsenic  equal  to  1-2  percent  of  the  platinum  in 
the  contact  material  will  destroy  the  catalytic  action  completely. 
In  view  of  the  enormous  volumes  of  gas  which  are  handled,  it  is 
easy  to  see  to  what  an  infinitesimal  value  the  arsenic  content 
must  be  reduced. 

By  cooling  the  burner  gases  slowly  and  bringing  them  into 
intimate  contact  with  sulphuric  acid,  it  was  found  possible  to 
obtain  a  gas  mixture  which  passed  all  ordinary  optical  and  chemi- 
cal tests  and  which  appeared  to  be  pure.  When  tested  on  the 
large  scale,  it  was  still  found  to  contain  arsenic.  The  gases  had 
been  cooled  in  long  iron  tubes  and  had  reacted  with  these  suffi- 
ciently to  give  traces  of  arsine.  This  difficulty  was  eliminated 
by  not  letting  the  gases  come  in  contact  with  the  iron.  When 
the  roaster  furnaces  were  run  to  maximum  capacity,  however, 
a  slight  cloud  was  formed  which  could  not  be  got  rid  of  and 
which  proved  to  consist  of  unburned  sulphur.  This  in  itself 
would  have  done  no  harm  had  not  the  particles  of  sulphur  con- 
tained some  arsenic.  It  therefore  proved  necessary  to  admit 
enough  air  so  as  to  be  sure  that  the  sulphur  was  burned  com- 
pletely. While  no  one  has  shown  that  the  presence  of  arsenic 
cuts  down  the  adsorption  by  platinum,  it  is  quite  certain  from 
Berliner's  experiments  on  palladium  and  hydrogen1  that  this 
must  be  the  case. 

In  American  practice  the  burner  gases  carry  arsenic,  lead 
sulphate,  silicon  tetrafluoride,  hydrochloric  acid,  sulphur,  and 
selenium.  Reese2  calls  the  first  two  permanent  poisons  and  the 
others  temporary  poisons.  When  working  with  platinum  on  a 
magnesium  sulphate  base,  it  is  possible  to  regenerate  the  contact 
material  after  it  has  been  poisoned  with  arsenic  by  heating  with 
hydrochloric  acid.  The  assumption  was  that  the  arsenic  was 
volatilized  as  the  trichloride.  Analysis  showed,  however,  that 
there  might  be  2-3  percent  of  arsenic  in  the  regenerated  catalyst 
without  its  having  any  apparent  effect.  It  must  be  that  the 

1  BERLINER:  Wied.  Ann.,  35,  804  (1888). 

2  Jour.  Soc.  Chem.  Ind.,  22,  351  (1903). 


ADSORPTION  OF  GAS  OR  VAPOR  BY  SOLID  51 

arsenic  had  agglomerated  in  some  way  so  that  it  did  not  coat 
the  platinum.  In  spite  of  Knietsch's  statement  to  the  contrary, 
it  is  possible  to  filter  out  all  the  solid  poisons. 

Under  ordinary  conditions  sulphur  dioxide  and  oxygen  com- 
bine very  slowly  and  sulphur  trioxide  dissociates  very  slowly  in 
the  absence  of  catalytic  agents.  In  presence  of  catalytic  agents 
sulphur  trioxide  begins  to  dissociate  perceptibly  at  about  400° 
and  is  practically  completely  dissociated  at  1000°.  At  first  sight 
it  would  therefore  seem  that  the  process  should  be  run  below  400°. 
The  time  factor  is  a  complicating  one,  however.  With  platinum 
as  catalytic  agent  the  reaction  begins  at  about  200°,  but  is 
relatively  slow  at  that  temperature,  and  under  commercial  con- 
ditions there  is  practically  no  reaction  below  400°.  It  has  proved 
best  technically  to  work  at  about  425°-450°,  the  increased  reaction 
velocity  compensating  for  the  decreased  chemical  efficiency. 
At  425°  the  conversion  is  98  percent,  dropping  to  91  percent  at 
500°.  Ferric  oxide  can  be  used  as  a  catalytic  agent;  but  the 
reaction  velocity  is  less  and  consequently  it  is  necessary  to  work 
at  a  higher  temperature  with  a  correspondingly  lower  chemical 
efficiency,  perhaps  60  percent.  Ferric  oxide  is  much  less 
susceptible  to  arsenic  poisoning  than  is  platinum,  presumably 
because  of  the  higher  temperature  at  which  it  acts.  Since 
arsenic  oxide  is  itself  a  catalytic  agent  when  free  from  iron,1 
there  must  be  temperatures  at  which  it  will  have  little  or  no 
deleterious  action.  If  one  could  make  a  ferric  oxide  catalyst 
which  would  work  at  as  low  a  temperature  as  platinum,  it  is 
probable  that  it  would  be  about  as  sensitive  to  arsenic  poisoning 
as  platinum.  It  is  entirely  a  question  of  the  way  in  which  the 
arsenic  precipitates  on  the  surface  of  the  catalytic  agent. 

The  heat  of  reaction  for  the  conversion  of  sulphur  dioxide  into 
sulphur  trioxide  is  given  as  22,600  cal  per  mol  of  gaseous  sulphur 
trioxide.  Since  this  heat  is  evolved  at  the  surface  of  the  contact 
material,  it  is  quite  a  serious  matter  to  keep  the  temperature 
within  the  proper  range.  The  technical  manufacture  of  sulphur 
trioxide  is  therefore  very  largely  an  engineering  problem  and  the 
recent  improvements  have  been  chiefly  changes  in  design2  so  as 
to  give  more  efficient  temperature  control. 

1  Cf.  BERL:  Zeit.  anorg.  Chem.,  44,  267  (1905). 

2  ZEISBERG:  Trans.  Am.  Electrochem.  Soc.,  36  (1919). 


52  APPLIED  COLLOID  CHEMISTRY 

HARGREAVES  SALT-CAKE  PROCESS 

The  Hargreaves  and  Robinson  process1  for  making  salt-cake 
was  established  on  a  practical  basis  in  1870.  The  upper  tem- 
perature limit  is  about  600°  owing  to  the  fusion  of  the  salt.  Origi- 
nally the  process  involved  no  catalytic  agent  but  consisted 
simply  in  the  passage  of  pyrites  gases  over  suitably  prepared 
salt  at  500°-550°,  the  reaction  being 

4NaCl  +  2S02  +  02  +  2H20  =  2Na2S04  +  4HC1 

In  1886  small  amounts  of  iron  oxide  were  added  as  a  catalytic 
agent.  Krutwig2  showed  that  without  ferric  oxide  about  28 
percent  of  the  sulphur  in  pyrites  was  converted  into  sodium 
sulphate  after  four  hours  at  a  red  heat.  With  ferric  oxide  a  70 
percent  yield  was  obtained  at  high  temperatures  and  90  percent 
at  lower  ones.  The  action  is  also  accelerated  by  cupric  oxide 
and,  since  1907,  0.1-1.0  percent  of  copper  or  iron  salts  or  both 
have  been  added  to  the  salt  before  the  latter  is  moulded,  the 
amount  added  depending  on  the  required  purity  of  the  product. 
When  the  salt-cake  is  to  be  used  in  glass  works,  only  a  small 
amount  of  the  catalytic  agent  can  be  added;  but  the  maximum 
amount  is  permissible  when  the  salt-cake  is  intended  for  the 
LeBlanc  soda  process.  Increasing  the  amount  of  catalytic 
agent  increases  the  reaction  velocity  and  lowers  the  temperature 
at  which  the  action  starts.  In  this  particular  process  there  is 
little -or  no  difficulty  with  the  poisoning  of  the  catalytic  agent 
because  the  oxides  are  not  used  over  and  over  again. 

DEACON  CHLORINE  PROCESS 

In  1845  Oxland3  discovered  that  chlorine  is  formed  to  some 
extent  when  a  mixture  of  hydrochloric  acid  and  oxygen  is  passed 
over  a  heated  porous  substance.  The  reaction  is  4  HC1  +  02  = 
2H2O  +  2C12;  but  the  reaction  does  not  proceed  fast  enough  to  be 

1  THORPE:  Dictionary  of  Applied   Chemistry,   6,  25   (1913);  JOBLING: 
Catalysis  and  its  Industrial  Applications,  32  (1916;;  RIDEAL  and  TAYLOR: 
Catalysis  in  Theory  and  Practice,  89  U919J. 

2  Recueil  Trav.  chim.  Pays-Bas,  16,  173  (1897). 

3  THORPE:  Dictionary  of  Applied   Chemistry,   2,   18   (1918);    JOBLING: 
Catalysis  and  its  Industrial  Applications,  26  (1916);  RIDEAL  and  TAYLOR: 
Catalysis  in  Theory  and  Practice,  99  (1919). 


ADSORPTION  OF  GAS  OR  VAPOR  BY  SOLID  53 

practicable  commercially.  In  1855  Vogel  treated  cupric  chloride 
alternately  with  hydrochloric  acid  and  oxygen.  In  1888  Deacon 
and  Hurter  put  the  process  on  a  successful  technical  basis  by 
passing  a  mixture  of  hydrochloric  acid  and  oxygen  over  pumice 
impregnated  with  cupric  chloride  which  acts  as  a  catalytic  agent. 
It  is  usually  assumed  that  this  is  not  a  case  of  adsorption  catalysis 
but  that  the  cupric  chloride  breaks  up  into  chlorine  and  cuprous 
chloride  which  latter  combines  with  oxygen  to  form  cupric 
oxy chloride,  CuO.CuCl2.  This  reacts  with  hydrochloric  acid 
to  form  cupric  chloride,  after  which  the  whole  series  of  reactions 
take  place  again  and  again.  There  is  no  real  proof  of  this1  and 
it  is  really  quite  as  probable  that  the  oxygen  does  react  direct 
with  the  hydrochloric  acid,which  is  what  it  must  do  when  there 
is  no  cupric  chloride  present.  The  catalytic  agent  is  very  sensitive 
to  the  effect  of  impurities,  such  as  sulphur  dioxide,  sulphur  trio- 
oxide,  and  arsenic.  It  is  because  of  this  sensitiveness  to  sulphur 
dioxide  that  the  hydrochloric  acid  from  the  Hargreaves  salt-cake 
process  cannot  be  used  in  the  Deacon  process.  Water  is  also 
bad  for  the  reaction.  The  gases  are  therefore  dried  at  40°  before 
entering  the  decomposer,  as  it  is  called.  The  temperature  at 
which  the  best  results  are  obtained  is  about  450°-460°.  Above 
this  temperature  the  cupric  chloride  volatilizes  too  rapidly  and 
at  lower  temperatures  the  reaction  velocity  is  too  low.  About 
two-thirds  of  the  hydrochloric  acid  is  converted  into  chlorine. 

CHANCE-GLAUS  PROCESS 

In  the  LeBlanc  process  for  making  sodium  carbonate,  calcium 
sulphide  is  a  by-product.  As  far  back  as  1837  Gossage2  patented 
a  process  for  using  this  tank  waste.  His  plan  was  to  treat  the 
calcium  sulphide  with  carbon  dioxide  and  then  to  burn  the 
resulting  hydrogen  sulphide  to  sulphur  according  to  the  equations, 
CaS  +  H20  +  C02  =  CaC03  +  H2S 

2H2S  +  02  =  2S  +  2H20 

Gossage  failed  because  he  could  not  get  a  sufficiently  concentrated 
CO2  to  start  with  or  a  sufficiently  concentrated  hydrogen  sulphide 
for  the  second  stage  of  the  process.  This  problem  was  solved  by 
Chance  in  1887. 

1  HURTER:  Jour.  Soc.  Chem.  Ind.  2,  106  (1883). 

2  JOBLING:  Catalysis  and  its  Industrial  Applications,  33  (1916). 


54  APPLIED  COLLOID  CHEMISTRY 

The  burning  of  the  hydrogen  sulphide  was  carried  out  accord- 
ing to  a  process  patented  by  Glaus  but  developed  further  by 
Chance.1  The  Glaus  kiln  consists  of  a  cylinder  provided  inter- 
nally with  a  grating  upon  which  rests  a  layer  of  broken  fire-brick, 
with  a  further  layer  of  bog-iron  ore  superimposed.  The  combus- 
tible mixture  enters  at  the  top,  passes  through  the  porous  layers, 
where  it  is  burned  to  sulphur  vapor  and  steam  with  but  little  sul- 
phur dioxide  and  sulphuretted  hydrogen  as  impurity,  and  is 
drawn  from  the  bottom  into  a  series  of  condensing  chambers, 
where  part  of  the  sulphur  vapor  condenses  to  liquid  sulphur  and 
the  remainder  as  flowers  of  sulphur  along  with  the  condensed 
steam.  The  reaction  is  exothermic,  so  that  no  external  heating 
is  necessary.  To  start  the  reaction,  a  few  shovelfuls  of  red-hot 
coal  are  thrown  on  to  the  oxide;  whilst  the  temperature  is  regulated 
by  controlling  the  speed  of  the  gas.  During  the  process  the 
ferric  oxide  becomes  transformed  into  pyrites,  but  the  pyrites 
so  formed  has  some  peculiar  property  attached  to  it  which  ordi- 
nary pyrites  do  not  possess,  for  the  latter  is  quite  useless  as  a 
catalyst.  When  once  the  necessary  temperature  has  been 
reached,  the  reaction  is  found  to  proceed  fairly  satisfactorily 
with  other  contact  material  such  as  broken  brick,  though  the 
working  temperature  then  becomes  somewhat  higher  than  when 
ferric  oxide  only  is  employed.  For  starting  the  reaction,  however, 
ferric  oxide  or  similar  contact  material  must  be  employed;  hence 
the  two  layers  in  the  Glaus  kiln. 

Salamon2  says  that  the  ferric  oxide  undergoes  no  change.  If 
that  is  so,  it  is  not  necessary  to  account  for  the  difference  in 
behavior  between  natural  pyrites  and  that  formed  in  the  Glaus 
kiln.  If  iron  sulphide  is  actually  formed,  the  alleged  difference 
in  behavior  is  undoubtedly  due  to  a  difference  in  surface  and  con- 
sequently of  adsorption.  The  natural  pyrites  is  of  course  only 
very  slightly  porous.  Rideal  and  Taylor3  consider  that  the  reaction 
takes  place  in  two  stages, 

Fe203  +  3H2S  =  2Fe  S  +  3H2O  +  S 
4FeS  +  302  =  2FeO  +  48. 

They  do  not  discuss  the  behavior  of  natural  pyrites. 

1  JOBLING:  Catalysis  and  its  Industrial  Applications,  33,  (1916). 

2  THORPE:  Dictionary  of  Applied  Chemistry,  6,  294  (1917). 

3  Catalysis  in  Theory  and  Practice,  112  (1919). 


ADSORPTION  OF  GAS  OR  VAPOR  BY  SOLID 


55 


HABER  AMMONIA  PROCESS 

In  the  contact  sulphuric  acid  process  it  was  found  that  sul- 
phur trioxide  dissociates  practically  completely  into  sulphur 
dioxide  and  oxygen,  at  1000°,  while  recombination  is  practically 
complete  at  400°  if  equilibrium  is  reached.  Matters  are  not  so 
favorable  in  the  case  of  the  synthesis  of  ammonia  from  nitrogen 
and  hydrogen.  At  1000°  ammonia  is  practically  completely 
dissociated  just  as  is  sulphur  trioxide;  but  at  550°  the  theoretical 
amount  of  recombination  at  atmospheric  pressure  is  about  0.08 
percent  instead  of  100  percent.  Increase  of  pressure  displaces 
the  equilibrium  so  that  more  is  formed  of  the  system  occupying 
the  lesser  volume,  namely  ammonia.  At  550°  and  200  atm.  the 
amount  of  ammonia  formed  is  almost  12  percent.  The  data1 
are  given  in  Table  XVIII. 


TABLE  XV11I. — EQUILIBRIUM  DATA  FOB  AMMONIA  SYNTHESIS 
Reaction  is  N2  +  3H2  =  2NH3 


Pressure 
in  atm. 

550° 

650° 

750° 

850° 

1000° 

Percentage  ammonia 

1 

0.077 

0.032 

0.916 

0.008 

0.006 

100 

0.7 

3.02 

1.54 

0.87 

0.54 

200 

11.9 

5.71 

2.99 

1.68 

1.07 

Though  the  yield  of  12  percent  is  low,  the  ammonia  can  be 
removed  as  ammonia  of  crystallization  by  ammonium  nitrate 
at  low  temperatures,2  a  fresh  supply  of  the  reacting  gases  can 
be  added  to  the  unchanged  nitrogen  and  hydrogen,  and  the  whole 
can  be  passed  repeatedly  through  the  reacting  chamber  until 
the  concentration  of  methane  and  argon  becomes  too  high.3  What 
catalytic  agent  to  use  is  a  question  of  first  importance.  The 
first  catalytic  agents  tried  were  nickel,  manganese,  iron  and 
chromium;  afterwards  experiments  were  made  with  osmium  and 

1  HABER:  Jour.  Soc.  Chem.  Ind.,  33,  53  (1914). 

2  THORPE:  Outlines  of  Industrial  Chemistry,  151  (1916). 

3RiDEAL  and  TAYLOR:  Catalysis  in  Theory  and  Practice,  241  (1919). 


56  APPLIED  COLLOID  CHEMISTRY 

uranium.1  While  metallic  osmium  and  uranium  carbide  are 
apparently  the  best  for  laboratory  work,  they  are  not  well 
adapted  for  technical  use.2  Judging  by  the  most  recent  patents, 
pure  iron3  is  the  catalyst  generally  employed  in  this  process. 
Using  this  material  and  with  the  aid  of  various  minor  improve- 
ments, it  is  said  to  be  found  possible  to  lower  the  working  pres- 
sure to  the  neighborhood  of  50  atmospheres.  In  all  cases,  it 
is  essential  in  the  interests  of  efficiency,  that  the  catalyst  be 
prepared  at  a  temperature  not  greatly  exceeding  600°.  Of 
the  remaining  substances  which  catalyze  the  reaction,  the  most 
interesting  are  molybdenum  and  molybdates,  tungsten  and  its 
alloys,  and  cerium  with  its  congeners.  Platinum,  on  the  other 
hand,  though  related  to  osmium,  possesses  but  a  slight  catalytic 
activity.  It  was  found  that  the  activity  of  the  catalysts  can  be 
increased  by  the  addition  of  certain  foreign  bodies.  These 
"  promoters, "  as  they  have  been  called,  include  various  metals, 
the  oxides,  hydroxides,  and  salts  of  the  alkali  and  alkaline  earth 
metals,  as  well  as  many  other  substances  of  the  most  varied 
nature.  In  many  cases  just  a  trace  of  the  contaminating  body 
enhances  considerably  the  activity  of  the  catalyst.  Moreover,  all 
the  catalysts  appear  to  possess  the  capacity  for  invigoration.4 
There  are  no  data  to  show  whether  these  so-called  promoters  change 
the  degree  of  adsorption  of  any  of  the  substances  concerned  in 
the  reaction  or  whether  they  prevent  a  possible  sintering  of  the 
catalytic  agent.  Mannich  and  Thiel5  state  that  charcoal  increases 
the  adsorbing  power  of  palladium  for  hydrogen.  Rideal  and 
Taylor6  consider  that  the  promoters  may  cause  the  relative 
adsorption  of  the  reacting  substances  to  approach  the  theoretical 
ratios  more  closely. 

The  contact  ammonia  process  is  very  susceptible  to  poisons. 
Among  the  harmful  substances  are  sulphur,  selenium,  tellurium, 
phosphorus,  arsenic,  boron,  and  their  compounds;  many  carbon 
compounds;  lead,  zinc,  bismuth,  tin,  etc.7  Such  minute  quanti- 

1  HABER:  Jour.  Soc.  Chem.  Ind.,  33,  54  (1914). 

2  JOBLING:  Catalysis  and  its  Industrial  Applications,  42  (1916). 

3  Cf.  JELLINEK:  Zeit.  anorg.  Chem.,  71,  121  (1911). 

4  PEASE  and  TAYLOR:  Jour.  Phys.  Chem.  24,  241  (1920). 

5  Jour.  Soc.  Chem.  Ind.,  35,  548  (1916), 

6  Catalysis  in  Theory  and  Practice,  29  (1919). 

7  JOBLING:  Catalysis  and  its  Industrial  Applications,  44  (1916). 


ADSORPTION  OF  GAS  OR  VAPOR  BY  SOLID  57 

ties  of  any  of  the  above-mentioned  substances  as  are  to  be  found 
almost  always  in  the  purest  commercial  products  and  so-called 
pure  gases  are  sufficient  to  diminish  the  catalytic  efficiency  very 
seriously.  For  instance,  an  impurity  of  0.01  percent  sulphur 
in  iron  renders  it  nearly  useless.  The  Haber  process  was  of 
enormous  value  to  the  Germans  during  the  war. 

OSTWALD  NITRIC  ACID  PROCESS 

In  1830  Kugelmann1  observed  that  ammonia  can  be  oxidized 
to  nitric  acid  in  presence  of  platinum.  About  eighty  years  later 
the  process  was  put  on  a  semi-commercial  basis  by  Ostwald.  The 
nominal  reaction  may  be  written  NH3  +  2O2  =  HNO3  +  H2O; 
but  the  real  reaction  is  the  formation  of  nitric  oxide,  which  is 
then  oxidized  to  nitrogen  peroxide,  and  afterwards  converted 
into  nitric  acid, 

4NH3  +  702  =  4NO  +  6H2O  +  2O2  =  4NO2  +  6H2O  =  2HNO3  + 

2HNO2  +  4H2O 

The  problem  is  complicated  by  the  apparently  simultaneous 
oxidation  to  nitrogen,  a  reaction  which  is  usually  written 

4NH3  +  3O2  =  2N2  +  6H2O 

It  is  more  likely  that  the  disturbing  reaction  is  an  oxidation  of 
ammonia  by  nitric  oxide,2  perhaps 

4NH3  +  6NO  =  5N2  +  6H20. 

Both  the  reactions  are  accelerated  by  platinum;  but,  fortunately, 
the  reaction  yielding  nitric  oxide  takes  place  much  more  rapidly 
than  the  reaction  giving  rise  to  nitrogen.  The  best  results  are 
obtained  by  keeping  the  gases  in  contact  with  the  catalytic 
agent  for  as  short  a  time  as  possible,  which  is  an  argument  for 
the  reaction  producing  nitrogen  being  a  secondary  one.  If 
the  two  reactions  took  place  simultaneously,  the  ratio  of  the  two 
sets  of  products  should  be  independent  of  the  rate  of  passage  of 
the  gaseous  mixture.  A  mixture  of  ammonia  with  ten  or  more 
volumes  of  air  is  passed  at  a  velocity  of  1-5  meters  per  second 

1  JOBLING:  Catalysis  and  its  Industrial  Applications,  45  (1916). 
2SABATiER  and  SENDERENS:  Comptes  rendus,  135,  278  (1902). 


58  APPLIED  COLLOID  CHEMISTRY 

through  a  plug  of  platinum  gauze,  1-2  cm  in  length  and  kept 
at  300°.  This  gives  a  period  of  contact  not  exceeding  0.01  sec. 
Under  favorable  conditions  an  85  percent  conversion  of  ammonia 
is  claimed.  Better  results  have  been  obtained  in  this  country,1 
with  several  layers  of  platinum  gauze,2  the  yield  running  92 
percent,  with  a  flow  of  250  cubic  feet  per  minute.  The  average 
concentration  of  ammonia  is  10.7  percent.  A  smooth  platinum 
gauze  is  not  effective.  After  some  time  it  becomes  roughened 
and  activated.  Contrary  to  what  Ostwald  had  believed,  a 
high  temperature,  say  above  825°,  is  better  than  a  lower  one. 
Phosphine  is  a  bad  poison,  having  a  distinct  effect  even  when 
present  three  parts  per  hundred  million,  but  cyanogen  compounds 
are  not  bad  because  they  oxidize  readily.  Iron  oxide,  grease,  oil 
or  tar  must  be  kept  away  from  the  platinum  gauze  and  a  new 
gauze  should  be  washed  with  pure  gasolene  or  ether  before  being 
activated.  Ammonia  from  any  source  can  be  used  though  Landis3 
claims  that  the  platinum  must  be  activated  anew  when  changing 
to  a  different  ammonia. 

SAJBATIER'S  HYDROGENATION  PROCESS 

Sabatier  and  Senderens4  discovered,  a  number  of  years  ago, 
that  reduced  nickel  will  cause  hydrogen  to  combine  with  ethyl- 
ene  or  acetylene  forming  ethane.  This  has  since  developed  into 
a  general  hydrogenation  process.5  The  principle  of  the  method 
is  a  simple  one.  The  vapor  of  the  substance  to  be  reduced  is 
mixed  with  hydrogen  and  passed  directly  over  specific  metallic 
catalysts  maintained  at  temperatures  usually  held  between  150° 
and  300°.  The  process  is  extremely  rapid,  since  complete  hydro- 
genation occurs  during  the  short  time  the  mixture  is  passing  over 
the  catalyst.  Rapidity,  however,  is  not  the  only  advantage,  for 
Sabatier  and  his  collaborators,  Senderens,  Mailhe,  and  others, 
who  have  worked  on  this  method  for  some  fifteen  years,  find  it 
to  be  one  which,  whilst  requiring  the  minimum  of  attention, 
furnishes  very  high  yields. 

LARSONS:  Jour.  Ind.  Eng.  Chem.  11,  541  (1919). 

2  PERLEY:  Jour.  Ind.  Eng.  Chem.  12,  5,  119  (1920). 

3  Trans.  Am.  Electrochem.  Soc.,  35,  300  (1919). 

4  Comptes  rendus,  124,  1358  (1897);  128,  1173  (1899). 

5  JOBLING:  Catalysis  and  its  Industrial  Applications,  68  (1916);  SABATIER: 
Die  Katalyse  in  der  organischen  Chemie,  44  (1914). 


ADSORPTION  OF  GAS  OR  VAPOR  BY  SOLID  59 

The  metals  which  are  suitable  as  catalysts  are  nickel,  cobalt, 
platinum,  iron  and  copper,  of  which  nickel  is  far  and  away  the 
most  active,  while  the  others  are  arranged  in  diminishing  activity. 
As  would  be  anticipated,  the  practicability  of  the  method  depends 
largely  upon  the  condition  in  which  the  metal  is  used.  It  must 
be  prepared  by  reduction  of  the  oxide,  the  activity  depending  upon 
the  nature  of  the  oxide  employed  and  the  temperature  at  which 
it  is  reduced.  In  general  the  greater  the  surface  exposed  the 
greater  the  activity,  so  that  the  conditions  must  be  carefully 
arranged  to  satisfy  this  requirement.  Reduction  at  a  low  tem- 
perature is  found  to  yield  a  metal  which  is  too  active  and  too 
sensitive  to  external  influences,  while  reduction  at  a  high  tempera- 
ture reduces  the  catalytic  action  almost  to  zero.  Consequently, 
a  suitable  intermediate  temperature  must  be  chosen.  Sabatier 
finds  that  the  best  nickel  for  the  purpose  is  produced  by  dissol- 
ving the  metal  in  nitric  acid,  calcining  the  nitrate  at  a  dull  red 
heat,  and  then  reducing  the  oxide  slowly  at  a  temperature  of 
about  300°-325°,  until  water  is  no  longer  evolved.  After  the 
nickel  has  been  prepared  as  described,  it  is  necessary  to  keep  it 
out  of  contact  with  the  air,  as  it  is  extremely  pyrophoric  and 
quickly  loses  its  activity  on  exposure. 

Weight  for  weight,  the  efficiency  of  pure  nickel  is  not  so  great 
as  that  which  has  had  its  active  surface  increased  by  the  aid  of 
some  suitable  carrier.  Hence  many  proposals  have  been  made  for 
combining  the  catalyst  with  a  great  variety  of  supporting  bodies, 
ranging  from  pumice  and  kieselguhr  to  charcoal  and  sawdust. 

The  materials  used,  whether  hydrogen,  metal,  or  organic  com- 
pound, must  be  as  pure  as  can  be  obtained,  for  any  impurity  is 
readily  adsorbed  by  the  catalyst  to  the  detriment  of  its  ac- 
tivity. Sulphur  and  its  volatile  compounds,  as  well  as  the  halo- 
gens,1 are  particularly  to  be  avoided  in  this  respect,  and,  to  a 
lesser  degree,  arsenic  and  phosphorus  with  their  volatile  com- 
pounds. Electrolytic  hydrogen,  or  hydrogen  of  equivalent 
purity,  appears  to  be  essential.  Even  then  the  catalyst  must, 
sooner  or  later,  succumb  to  the  toxic  effects  of  minute  impurities 
impossible  to  remove;  in  consequence,  the  desirability  of  fre- 
quent renewals  needs  emphasis. 

1  The  nickel  oxide  must  not  be  prepared  from  the  chloride  because  of  the 
great  difficulty  of  washing  out  the  last  traces  of  adsorbed  chloride. 


60  APPLIED  COLLOID  CHEMISTRY 

Of  practical  importance  is  the  preparation  of  numerous  arti- 
ficial perfumes  by  the  aid  of  this  reaction.  Moreover,  it  permits 
of  the  production  of  cyclohexanol  and  p-methyl  cyclohexanol, 
easily  obtained  from  phenol  and  p-cresol  respectively,  which  are 
used  in  the  manufacture  of  isoprene  and  butadiene,  and  seem, 
therefore,  destined  to  play  an  important  part  in  the  synthesis 
of  rubber. 

Probably  the  most  important  technical  application  of  Saba- 
tier's  hydrogenation  process  is  to  the  conversion  of  liquid  and 
unsaturated  fatty  acids  such  as  oleic  acid1  into  solid  and  saturated 
acids  such  as  stearic  acid.  As  the  acid  is  not  vaporized,  this 
process  does  not  belong  strictly  speaking  under  the  general 
head  of  gas  reactions  accelerated  by  solid  catalytic  agents,  so  I 
merely  refer  to  it  because  it  is  a  further  development  of  the  same 
principle. 

SABATIER'S  DEHYDEOGENATION  PROCESS 

Attention  has  already  been  drawn  to  the  fact  that  alcohols 
may  split  off  hydrogen  or  water  at  a  given  temperature,  depend- 
ing on  the  nature  of  the  catalytic  agent.  Finely  divided  metals,2 
such  as  copper,  cobalt,  nickel,  iron,  platinum,  and  palladium 
catalyze  almost  exclusively  the  dehydrogenation  process.  So 
also  do  some  anhydrous  oxides,  chief  among  which  are  the  lower 
oxides  of  manganese,  tin,  uranium,  molybdenum,  vanadium,  and 
cadmium,  though  the  activity  of  these  is  less  than  that  of  the 
metals.  The  dehydrbgenation  of  alcohols,  both  primary  and 
secondary,  is  most  readily  effected  by  reduced  copper.  If  a 
fatty  alcohol  be  passed  over  this  catalyst  at  200°-300°,  a  yield  of 
at  least  50  percent  aldehyde  or  ketone  can  usually  be  condensed. 

Cyclic  alcohols  may  be  dehydrogenated  in  a  similar  way. 
Borneol,  for  instance,  can  be  transformed  into  camphor  by  pas- 
sage over  reduced  copper  at  150°,  whilst  geraniol  under  similar 
conditions  gives  citral.  The  former  fact  is  utilized  industrially 
in  the  treatment  of  certain  camphors  which  are  sold  very  cheaply 

*Cf.  ELLIS:  Jour.  Soc.  Chem.  Ind.,  31,  1155  (1912);  SHAW:  Ibid.,  33, 
771  (1914);  ELLIS:  Jour.  Ind.  Eng.  Chem.,  8,  886  (1916);  ELLIS:  The  Hydro- 
genation of  Oils,  2nd  Ed.,  108  (1918). 

2  JOBLING:  Catalysis  and  its  Industrial  Applications,  82  (1916). 


ADSORPTION  OF  GAS  OR  VAPOR  BY  SOLID  61 

on  account  of  their  large  content  of  borneol.  As  the  usual  proc- 
ess for  removing  borneol  by  means  of  nitric  acid  is  very  slow 
and  unsatisfactory,  the  above  dehydrogenation  method,  which 
yields  nearly  pure  camphor,  has  found  ready  application  in  this 
field. 

Of  the  other  catalysts,  cobalt,  iron,  platinum,  as  well  as  the 
above-mentioned  oxides,  are  less  advantageous.  Zinc  is  stated 
to  possess  high  activity.  Nickel  is  ruled  out  on  account  of  the 
violence  of  its  reaction,  for  it  pushes  decomposition  too  far. 

The  most  important  of  the  industrial  applications  of  this 
reaction  is  the  preparation  of  formaldehyde  from  methyl  alco- 
hol.1 The  earliest  recorded  production  of  formaldehyde  is  to 
be  found  in  the  classic  experiment  of  Hofmann's  (1867),  in  which 
a  red-hot  coil  of  platinum  wire  suspended  over  methyl  alcohol 
in  a  beaker  continued  to  glow  there  so  long  as  it  is  in  contact  with 
a  mixture  of  the  vapor  and  air.  This  method  forms  the  basis  of 
most  of  the  subsequent  attempts  and  of  the  present  commercial 
process.  Air  is  driven  through  wood  spirit  kept  at  a  temperature 
suitable  for  saturation  and  the  mixture  then  passed  into  the 
catalyzer,  usually  copper  gauze. 

In  the  so-called  oxidation  of  methyl  alcohol,  the  primary 
reaction  is  the  splitting-off  of  hydrogen2  and  the  practically  si- 
multaneous depolarization  by  oxygen, 

2CH2OH  +  O2  =  2HCHO  +  4H  +  02  =  2HCHO  +  2H20. 

Hofmann3  found  that  osmium  tetroxide  oxidizes  alcohol  to 
aldehyde  but  not  to  acetic  acid,  acting  apparently  as  a  carrier  for 
oxygen.  It  is  probable  that  we  have  here  a  catalytic  splitting 
of  the  alcohol,  the  nascent  hydrogen  reacting  with  the  osmium 
tetroxide.  Wieland4  considers  that  the  contact  sulphuric  acid 
process  really  involves  a  dehydrogenation,  the  reactions  being: 

S02  +  H20  =  H2S03 

H2S03  =  S03  +  Hi 
2H2  +  O2  =  2H2O 

1  THORPE:  Dictionary  of  Applied  Chemistry,  2,  592  (1912). 
2L,EBLANC  and  PLASCHKE:  Zeit.  Elektrochemie,  17,  47  (1911). 
3  Ber.  deutsch.  chem.  Ges.  46,  1657,  2854  (1913). 
*  Ibid.  45,  685  (1912). 


62  APPLIED  COLLOID  CHEMISTRY 

SURFACE  COMBUSTION 

The  subject  of  surface  combustion  has  been  developed  by  Bone 
in  England1  and  by  Lucke  in  America.  Bone  began  by  a  study 
of  the  combination  of  hydrogen  and  oxygen  in  contact  with  hot 
surfaces  of  porcelain,  magnesia,  silver,  gold,  platinum,  nickel, 
calcined  spathic  iron  ore  (containing  79.0  percent  ferric  oxide 
and  14.5  percent  manganous  oxide),  nickelous  oxide,  and  copper 
oxide.  With  each  of  these  surfaces  the  rate  is  proportional  to 
the  pressure  of  the  dry  gas  when  electrolytic  gas  is  used,  is  ap- 
proximately proportional  to  the  partial  pressure  of  hydrogen  when 
either  gas  is  present  in  excess,  and  is  increased  by  a  previous 
treatment  with  hydrogen.  In  the  case  of  porcelain,  Bone  decides 
that  porous  porcelain  occludes  or  condenses  both  hydrogen  and 
oxygen  at  rates  which  depend  to  some  extent  upon  the  physical 
condition  and  past  history  of  the  surface.  While  the  process 
is  extremely  rapid  in  the  case  of  oxygen  and  the  surface  layer  is 
soon  saturated,  the  occlusion  of  hydrogen  is  slower  and  the  limit 
of  saturation  much  higher.  Combination  between  the  occluded 
gases  occurs  at  a  rate  either  comparable  with,  or  somewhat  faster 
than,  the  rate  at  which  the  film  of  occluded  oxygen  is  renewed, 
but  considerably  faster  than  the  rate  of  occlusion  of  hydrogen. 

In  later  papers  Bone2  showed  that  all  incandescent  substances 
are  capable  of  accelerating  gaseous  combustion;  and  to  an  approx- 
imately equal  degree,  for  the  wide  differences  between  the 
catalytic  powers  of  various  surfaces  at  low  temperatures  gradu- 
ally diminish  as  the  temperature  rises,  until  at  incandescence 
they  practically  disappear.  If  an  explosive  mixture  of  a  combus- 
tible gas  and  air  is  pressed  through  the  pores  of  a  granular,  high- 
melting  solid,  and  if  the  issuing  gas  be  lighted,  it  will  speedily 
raise  the  granular  solid  to  incandescence  if  the  conditions  are 
right,  after  which  the  heated  surface  will  accelerate  the  rate  of 
combustion  so  much  that  the  whole  of  the  gas  will  be  burned  at 
the  surface  of  the  solid  and  we  shall  have  what  has  been  called 
flameless  combustion.  As  a  matter  of  fact  there  is  flame,  but 
the  flame  is  so  short  and  the  light  from  the  incandescent  solid 
so  intense  that  there  seems  to  be  no  flame.  The  combustion 

1  Phil.  Trans.,  206A,  1  (1906);  Jour.  Franklin  Inst.,  173,  101  (1912). 

2  Summarized  in  Ber.  deutsch.  chem.  Ges.,  46,5  (1913);  JOBLING:  Catalysis 
and  its  Industrial  Applications,  51  (1916). 


ADSORPTION  OF  GAS  OR  VAPOR  BY  SOLID  63 

takes  place  only  at  the  surface  layer  of  the  granular  solid  but  all 
over  the  surface  and  not  as  flames  between  each  solid  particle. 
The  advantages  of  this  surface  combustion  are  an  intense  and  a 
localized  heat,  combustion  with  minimum  excess  of  air,  and  the 
development  of  a  large  amount  of  the  heat  as  radiant  energy. 
By  passing  the  explosive  mixture  through  a  porous  diaphragm 
we  get  an  incandescent  surface  which  can,  if  need  be,  be  held 
glowing  side  down  over  a  solution  which  is  to  be  evaporated. 
In  this  way,  it  is  possible,  for  instance,  to  evaporate  a  water- 
glass  solution  without  danger  of  bumping.  The  sodium  silicate 
forms  as  a  solid  film  on  the  surface  of  the  solution  and  can  be 
removed  from  time  to  time.  This  form  of  heating  has  proved 
very  successful  in  making  candy.1 

The  granular  mass  may  be  piled  in  round  a  crucible  or  around 
boiler  tubes  giving  an  extremely  high  efficiency.  The  maximum 
temperature  depends  on  the  working  conditions.  The  choice  of 
refractory  material  is  determined  by  the  temperatures  involved. 
Under  ordinary  conditions  calcined  fire-clay,  ganister,  etc.,  are 
used;  but  when  high  temperatures  are  in  question,  practically 
all  solids  are  eliminated  except  calcined  magnesia  and  carborun- 
dum. Attempts  are  now  being  made  to  substitute  an  atomized 
liquid  fuel  for  a  gaseous  fuel.  It  is  stated  that  the  most  success- 
ful applications  of  surface  combustion  to  large  scale  work  are 
where  the  heat  is  radiated  very  quickly  so  that  the  temperature 
of  the  refractory  does  not  rise  unduly — with  steam  boiler  tubes 
packed  in  refractory  material  for  instance.2  Surface  combustion 
furnaces  have  proved  very  useful  in  annealing  shell. 

Since  an  explosive  gas  mixture  is  used,  it  is  necessary  to  keep 
the  flame  from  working  back  and  burning  in  the  mass  of  the 
refractory  instead  of  on  the  surface.  Bone  does  this  by  regulat- 
ing the  rate  of  flow  of  the  gaseous  mixture.  Lucke3  has  described 
several  ways  of  doing  this  by  special  construction  of  the  heater. 

WELSBACH  GAS  MANTLE 

The  invention  of  the  gas  mantle  made  it  possible  for  gas  to 
continue  to  compete  with  electricity  as  an  illuminant.  The 

1  SEAQER:  Jour.  Franklin  Inst.,  177,  367  (1914). 

2  Reports  of  Progress  of  Applied  Chemistry,  1917,  26. 

3  Jour.  Ind.  Eng.  Chem.,  5,  801  (1913). 


64  APPLIED  COLLOID  CHEMISTRY 

best  results  are  obtained  when  the  Welsbach  gas  mantle  consists 
of  99  percent  thoria  and  1  percent  ceria.1  Pure  thoria  gives 
a  relatively  poor  light,  but  the  continuous  addition  of  ceria 
gradually  increases  the  luminosity  to  a  maximum  of  ten-fold 
when  1  percent  is  present.  Any  further  addition  then  dimin- 
ishes the  luminosity  until,  with  10  percent  the  difference  in 
effect  is  inappreciable.  In  present-day  manufacture,  the  cotton, 
silk  or  ramie  web  is  impregnated  with  a  25-33  percent  aqueous 
solution  of  thorium  and  cerium  nitrates  in  the  proportion  of 
99:1,  together  with  a  little  hardening  medium,  e.g.,  beryllium 
nitrate,  if  necessary. 

We  are  dealing  with  surface  combustion  both  in  the  case  of  the 
lime  light  and  of  the  Welsbach  mantle,  as  is  shown  by  the  rapid 
decay  of  the  light  of  the  lime  light  owing  to  sintering2  and  the 
similar  though  much  slower  decay  of  the  light  of  the  Welsbach 
mantle.  Mantles  of  magnesia  and  lanthana  give  a  good  light 
when  new;  but  the  light  decreases  as  the  surface  becomes  glassy. 
While  a  thoria  mantle  reaches  a  high  temperature,3  its  emissive 
power  is  low.  Ceria  has  a  high  emissive  power  in  the  visible 
spectrum;  but  the  energy  of  radiation  is  so  great  that  the  temper- 
ature of  a  ceria  mantle  does  not  rise  high  enough.  Starting  with 
thoria  and  increasing  the  amount  of  ceria  increases  the  emissive 
power  but  lowers  the  temperature  of  the  mantle.  The  maximum 
luminous  efficiency  is  obtained  when  the  rise  of  visible  emission 
due  to  the  ceria  is  balanced  by  the  drop  in  the  temperature  of  the 
mantle  due  to  the  increased  radiation.  The  ceria  may  form  a 
solid  solution  with  the  thoria  but  there  seem  to  be  no  data  to 
show  whether  equilibrium  is  actually  reached  in  the  mantle. 

1  JOBUNG:  Catalysis  and  its  Industrial  Applications,  62  (1916). 

2  STEINMETZ:  Radiation,  Light  and  Illumination,  92  (1909). 

3IvES,  KINGSBURY  and  KARRER:  Jour.  Franklin  Last.,  186,  401,  585 
(1918). 


CHAPTER  II 

ADSORPTION  OF  VAPOR  BY  LIQUID  AND  OF  LIQUID  AND 
SOLID  BY  SOLID  AND  LIQUID 

ADSORPTION  OF  VAPOR  BY  LIQUID 

Campbell1  believes  that  one  source  of  error  in  the  method  of 
determining  molecular  weights  by  the  so-called  air-bubbling 
method  is  due  to  the  formation  of  a  film  of  adsorbed  air  on  the 
surface  of  the  solution  which  interferes  with  the  evaporation. 
He  deduced  the  existence  of  an  adsorbed  gas  film  from  his  quanti- 
tative measurements  on  evaporation.  The  existence  of  such  a 
film  is  shown  directly  by  Rayleigh's  experiments  on  liquid  jets.2 
A  vertical  jet  of  water  or  any  liquid  slows  up  and  breaks  into 
drops  when  the  head  is  sufficiently  great  relatively  to  the  dia- 
meter of  the  jet.  These  drops  scatter  because  they  rebound 
when  they  strike  one  another.  If  a  feebly  electrified  body  is 
brought  close  to  the  jet,  the  jet  becomes  coherent.  Under  more 
powerful  electrical  action  the  scattering  of  the  drops  becomes 
even  greater  than  when  there  is  no  electrification.  What  happens 
is  that  there  is  formed  round  each  drop  a  film  of  adsorbed  air 
which  prevents  two  drops  from  coalescing  when  they  collide. 
When  the  drops  are  electrified,  the  air  film  is  removed  to  such  an 
extent  that  the  drops  come  more  nearly  in  contact  and  coalesce. 
With  higher  electrification,  a  marked  charging  of  the  drops  takes 
place  and  the  repulsion  of  the  like  charges  causes  the  drops  to 
scatter.  Nipher3  states  that  drops  of  1  mm  radius  do  not  attract 
each  other  when  charged  to  0.0031  volt.  That  there  is  an  air 
film  between  the  drops  can  also  be  shown  by  letting  two  jets  im- 
pinge under  such  conditions  that  they  bound  apart.  It  was 
found  that  whenever  the  jets  rebounded,  the  electrical  insulation 
was  practically  perfect.  Rayleigh  found  that  presence  of  dust 

1  Trans.  Faraday  Soc.,  10,  197  (1915). 

2  RAYLEIGH:  Proc.  Roy.  Soc.,  28,  40G;  29,  71   (1879);  34,  130  (1882). 

3  Science,  34,  442  (1911). 

5  65 


66  APPLIED  COLLOID  CHEMISTRY 

or  addition  of  a  drop  of  milk  had  a  marked  effect  on  the  ease 
with  which  the  drops  coalesce.  The  reason  for  this  has  not  been 
worked  out. 

In  Worthington's  experiments  on  splashes1  the  adsorbed  air 
film  plays  an  important  part.  A  drop  of  water  may  fall  on  a 
sheet  of  water  in  such  a  way  that  the  drop  passes  below  the  sur- 
face, becoming  completely  submerged,  and  then  emerges  at  the 
head  of  a  column  of  adherent  liquid  with  the  upper  portion 
apparently  un wetted  by  the  liquid  with  which  it  has  been  covered. 
When  the  water  drops  from  an  oar  on  the  recover,  we  often  get 
what  are  known  as  rolling  drops2  which  depend  on  the  presence 
of  adsorbed  air  films  and  which  are  also  sensitive  to  the  presence 
of  electrified  substances. 

If  the  surface  of  a  sheet  of  water  is  covered  with  oil  of  with 
a  scum  of  oily  soot  or  of  plankton  organisms,  a  mist  or  fog  may 
deposit  drops  on  the  surface  which  will  be  extremely  persistent. 
This  phenomenon  is  not  at  all  uncommon  at  Lake  Mendota 
at  Madison  and  has  frequently  been  observed  elsewhere.3 

Since  soap-bubbles  are  hollow  drops,  they  should  behave  in 
some  respects  like  liquid  drops,  though  not  necessarily  like  drops 
of  a  pure  liquid.  It  is  possible  to  press  two  soap  bubbles  together 
with  considerable  force  without  causing  them  to  coalesce;4 
but  if  they  are  electrified  slightly,  they  coalesce  readily  without 
bursting. 

It  is  interesting  to  note  that  water  will  apparently  remove  a 
film  of  adsorbed  air  much  more  readily  from  a  polished  surface 
of  ivory  or  marble  than  it  will  from  a  water  surface;5  but  we  do 
not  know  whether  this  is  true  for  all  solids.  Sakulka6  obtained 
somewhat  different  results  with  lead  shot,  but  this  may  have 
been  because  they  were  not  polished. 

If  a  vapor  is  adsorbed  by  a  liquid,  it  will  condense  to  a  liquid 
film  at  a  temperature  above  that  of  the  normal  dew-point.  Can- 

1  WORTHINGTON:  Proc.  Roy.  Soc.,  34,  217  (1882). 

2  OSBORNE  REYNOLDS:  Chem.  News,  44,  211  (1881);  KAISER:  Wied.  Ann., 
53,  681  (1894). 

3JuDAY:  U.  S.  Monthly  Weather  Review,  44,  65    (1916);  NAKAMURA: 
Jour.  Meteorological  Soc.,  Japan,  36  (1917). 
*  BOYS:  Phil.  Mag.  25,  409  (1888). 
6  WORTHINGTON:  Proc.  Roy.  Soc.,  34,  217  (1882). 
6  Drude's  Ann.,  25,  892  (1908), 


ADSORPTION  OF  LIQUID  BY  SOLID  67 

tor1  realized  this  with  water  and  mercury  and  Clark2  with  olive 
oil  which  is  not  a  well-defined  and  reproducible  liquid. 

ADSORPTION  OF  LIQUID  BY  SOLID 

If  a  liquid  is  adsorbed  at  a  solid  surface,  it  forms  a  liquid  film 
and  we  say  that  it  wets  the  solid.  If  a  liquid  is  not  adsorbed 
by  a  solid,  it  does  not  wet  the  latter.  There  has  been  a  good 
deal  of  question  whether  the  liquid  meets  the  wall  at  a  definite 
angle,  the  contact  angle,  or  whether  the  contact  angle  is  zero. 
Lord  Rayleigh3  states  definitely  that  in  his  opinion  the  contact 
angle  is  zero,4  and  this  must  be  true  if  we  consider  that  the 
liquid  is  adsorbed  by  the  solid.  The  formation  of  a  liquid  film 
over  the  surface  of  a  wetted  solid  accounts  for  the  generally 
accepted  fact  that  the  rise  of  a  liquid  in  a  capillary  tube  is  inde- 
pendent of  the  nature  of  the  walls  of  the  tube.  This  has  always 
seemed  a  very  improbable  state  of  things  and  one  that  could  only 
be  justified  by  the  fact  that  it  was  so.  The  whole  thing  becomes 
quite  simple  when  we  consider  that  the  rising  liquid  does  not 
come  in  contact  with  the  walls  of  the  capillary  tube  at  all.5  We 
are  really  dealing  with  the  rise  of  a  liquid  in  a  liquid  tube  and  it 
makes  no  difference  what  material  is  used  to  support  the  walls 
of  the  liquid  tube.  That  this  is  the  real  explanation  may  be  seen 
from  the  fact  that  concordant  results  are  not  obtained  when  a 
liquid  is  allowed  to  rise  in  a  dry  tube.  To  get  good  results  it  is 
important  to  immerse  the  tube  in  the  liquid  and  then  to  raise  it. 
Bigelow  and  Hunter6  claim  that  the  nature  of  the  walls  does  have 
an  effect  on- the  capillary  rise;  but  their  experiments  were  not 
done  in  a  capillary  tube. 

On  the  assumption  that  the  contact  angle  is  zero,  a  thin  film  of 
liquid  must  rise  to  the  top  of  the  tube,  irrespective  of  the  height 
and  diameter  of  the  tube.  This  has  been  recognized  explicitly 
by  Lord  Rayleigh.7  When  the  liquid  does  not  wet  the  solid, 

1  Wied.  Ann.,  66,  492  (1895). 

2  Proc.  Am.  Acad.,  41,  361  (1906). 

3  Scientific  Papers,  3,  393  (1902). 

*  Cf.  HARKINS  and  BROWN:  Jour.  Am.  Chem.  Soc.,  41,  522  (1919). 
8  RAYLEIGH:  Scientific  Papers,  3,  421  (1902). 

6  Jour.  Phys.  Chem.,  15,  367  (1911). 

7  Scientific  Papers,  3,  523  (1902). 


68  APPLIED  COLLOID  CHEMISTRY 

mercury  and  glass  for  instance,  the  assumption  of  a  zero  contact 
angle  leads  to  at  least  one  awkward  conclusion.  The  upper  part 
of  the  meniscus  is  certainly  not  in  contact  with  the  glass  and 
therefore  none  of  the  mercury  is  in  contact  with  the  glass.  If  we 
have  a  glass  tube,  infinitely  long,  closed  at  the  bottom,  and  filled 
with  mercury,  we  must  conclude,  if  we  are  going  to  be  strictly 
logical,  that  the  column  of  mercury  does  not  touch  the  glass  at 
any  point  and  that  it  stands  alone,  surrounded  on  all  sides  by  a 
film  of  air,  mercury  vapor,  or  something  which  is  not  glass. 
Personally,  I  feel  no  desire  to  be  so  logical  as  that.  It  seems  to 
me  probable  that  at  some  point  the  force  of  gravity  squeezes  out 
all  the  vapor  layer  and  that  the  mercury  does  come  in  contact 
with  the  glass.  This  is  the  more  certain  because  we  know  that 
the  mercury  sometimes  tends  to  hang  in  a  freshly-made  barometer 
and  this  could  not  happen  if  the  mercury  were  not  in  contact 
with  the  glass. 

For  a  liquid  to  wet  a  solid  in  the  presence  of  air,  the  liquid  must 
be  adsorbed  more  strongly  than  the  air  and  must  displace  it. 
Hofmann1  gives  a  number  of  details  as  to  the  slowness  with  which 
some  liquids  displace  adsorbed  air  from  certain  solids.  In  some 
cases  heating  is  necessary  to  get  quick  results.  Lycopodium 
powder  is  distinctly  difficult  to  wet.  Gum  arabic  adsorbs  air 
strongly  and  is  not  wetted  readily  by  water.  If  ground  with 
alcohol,  the  alcohol  displaces  the  air  and  then  the  gum  is  wetted 
more  easily  by  water.  The  floating  of  metallic  powders  or  of 
pieces  of  metal  on  water  is  due  to  the  slowness  with  which  water 
wets  them.  The  Cape  Cod  lighter  consists  of  a  por-ous  stone  set 
in  a  brass  handle.  When  this  is  dipped  into  a  can  of  kerosene, 
the  kerosene  is  sucked  rapidly  into  the  pores  displacing  the  air. 
The  light  is  taken  out,  lighted,  and  placed  under  the  logs  of  a 
wood  fire.  The  adsorbed  kerosene  burns  for  about  ten  minutes, 
enabling  one  to  do  away  with  paper  and  even  kindling  wood. 
Another  interesting  application  of  the  adsorption  of  a  liquid  by  a 
solid  is  the  case  of  solidified  bromine.2  It  consists  of  siliceous 
earth  saturated  with  bromine,  which  can  be  driven  out  by  heat. 

Of  more  technical  importance  is  the  adsorption  of  fused  magne- 

1  Zeit.  phys.  Chem.,  83,  385  (1913). 

2  BRAND:  Zeit.  anal.   Chem.,  26,  222;  Jour.  Chem.  Soc.,  52,  688  (1887). 
Cf.  THORP:  Outlines  of  Industrial  Chemistry,  251  (1916). 


ADSORPTION  OF  LIQUID  BY  SOLID  69 

tite  by  chromite.1  The  simple  form  of  magnetite  arc  consists  of 
a  lower,  negative,  magnetite  terminal  and  an  upper  non-consuming 
terminal  of  copper,  of  such  size  that  it  does  not  get  so  hot  as  to 
oxidize  or  evaporate,  but  sufficiently  hot  to  avoid  condensation 
of  magnetite  vapor.  When  the  arc  is  struck,  a  pool  of  melted 
magnetite  forms  on  the  surface  of  the  magnetite  terminal  and  the 
arc-conductor  issues  from  a  depression  in  this  melted  pool.  The 
straight  magnetite  arc  flickers  badly;  but  this  can  be  overcome 
by  mixing  the  magnetite  with  some  more  refractory  material, 
such  as  chromite.  The  chromite  remains  solid  and  holds  the 
melted  magnetite  as  a  sponge  does  water.  In  this  way,  these  arcs 
have  been  made  steady  and  thereby  suitable  for  industrial  use. 

We  are  quite  familiar  with  the  fact  that  glass  must  be  cleaned 
before  it  can  be  wetted  satisfactorily  by  water;  but  it  is  not  so 
well  known  that  platinum  is  only  wetted  by  mercury  under  special 
conditions.2  "Mercury,  which  holds  even  a  very  little  sodium 
in  solution,  has  the  power  of  wetting  platinum  in  a  very  re- 
markable manner.  The  appearance  of  the  platinum  is  quite 
similar  to  that  presented  by  amalgamable  metals  in  contact  with 
mercury.  But  the  platinum  is  in  no  wise  attacked.  Further, 
the  amalgam  may  be  washed  off  by  clean  mercury,  and  the  latter 
will  also  continue  to  adhere  closely  to  the  platinum.  All  the 
phenomena  of  capillarity  are  presented  between  the  two.  The 
surface  of  the  mercury  in  a  platinum  cup  so  prepared  is  quite 
concave;  and  a  basin  of  mercury  may  be  emptied  if  a  few  strips 
of  similarly  prepared  platinum  foil  be  laid  over  its  edge — just 
as  a  basin  of  water  may  be  emptied  by  strips  of  paper  or  cloth, 
and  under  the  same  condition,  namely  that  the  external  limb  of 
such  capillary  siphon  be  longer  than  the  internal  one.  The 
sodium  removes  the  oxide  film  and  permits  the  mercury  to  come 
in  contact  with  the  platinum." 

It  is  found  that  iron,  copper,  bismuth,  and  antimony  are  also 
wetted  by  mercury  if  their  surfaces  are  first  touched  with  sodium 
amalgam. 

A  few  years  later  a  technical  application  of  this  observation 
was  made.  Kirchman3  reports  that  sodium  amalgam  .  can  be 

1  STEINMETZ:  Radiation,  Light,  and  Illumination,  110  (1909). 

2  GUTHRIE:  Proc.  Roy.  Soc.,  13,  458  (1864). 

3  Jour.  Chem.  Soc.,  26,  418  (1873). 


70  APPLIED  COLLOID  CHEMISTRY 

used  advantageously  in  simplifying  the  method  of  dry-gilding 
iron.  Surfaces  of  iron  and  similar  metals,  even  though  oxidized, 
are  immediately  amalgamated  by  simply  rubbing  with  amalgam. 
Gold  chloride  in  concentrated  solution  is  then  applied  quickly 
on  the  amalgamated  surface,  and  the  mercury  volatilized  by  the 
heat  of  lamp  or  fire.  A  very  uniform  gilding  is  thus  obtained, 
permitting  a  high  polish.  Corresponding  results  are  obtained 
with  silver  and  platinum  salts. 

Wald1  has  pointed  out  that  there  must  be  a  lowering  of  the 
vapor  pressure  of  a  liquid,  when  a  liquid  wets  a  solid.  Conse- 
quently, if  water  really  wets  ice,  or  if  water  rises  in  a  capillary 
tube  made  of  ice,  the  vapor  pressure  of  dry  ice  will  differ  from  that 
of  wetted  ice  at  the  same  temperature.  This  point  is  not  taken 
into  account  in  books  on  the  phase  rule  because  effects  due  to 
capillarity,  gravity,  etc.,  are  usually  ruled  out  explicitly. 

The  adsorption  of  a  liquid  film  may  introduce  quite  an  error 
in  the  determination  of  the  specific  gravity  of  bodies  when  they 
are  weighed  in  a  highly  divided  state.  As  long  ago  as  1848  Rose2 
pointed  out  that  platinum  in  the  state  of  foil  has  a  specific  gravity 
between  21  and  22  while  a  value  above  26  was  found  for  platinum 
precipitated  from  the  chloride  by  sodium  carbonate  and  sugar. 
Similar  though  less  extreme  differences  were  found  with  gold, 
silver,  and  barium  sulphate.  This  can  be  accounted  for  if  we 
assume  that  the  powder  is  not  weighed  alone  in  water  but  in 
conjunction  with  a  film  of  condensed  water.3 

It  was  Edward  Lear  who  wrote  about  the  Jumblies  who  went 
to  sea  in  a  sieve.  He  considered  that  a  nonsense  rhyme;  but 
modern  science  is  almost  equal  to  the  problem,  for  we  can  carry 
water  in  a  sieve.  If  we  take  a  fine-meshed  metal  sieve  and  oil 
the  meshes,  air  can  still  pass  readily  through  the  holes;  but  water 
does  not  wet  the  wires  and  consequently  surface  tension  pre- 
vents it  from  passing  through  the  holes.4  Gasolene  may  be 
freed  from  water  by  filtering  through  a  200  mesh  metal  sieve. 
We  can  carry  water  in  such  a  sieve5  if  we  do  not  fill  it  too  full; 

1  Zeit.  phys.  Chem.,  7,  514  (1891). 

2Pogg.  Ann.,  73,  1  (1848);  Jour.  Chem.  Soc.,  1,  182  (1849). 

3  See  however  JOHNSTON  and  ADAMS:  Jour.  Am.  Chem.  Soc.,  34,  563 
(1912), 

4  CHWOLSON:  Lehrbuch  der  Physik,  1  III,  613  (1907). 
6  CAMPBELL:  Jour.  Ind.  Eng.  Chem.,  6,  762  (1919). 


ADSORPTION  OF  LIQUID  BY  SOLID  71 

but  an  oiled  sieve  is  not  a  perfect  substitute  for  a  pail.  This 
principle  has  received  an  important  technical  application  in  the 
shower- proofing  of  textile  fabrics.1  The  fibers  are  coated  with 
some  water-repellent  hydrocarbon,  wax,  or  salt,  in  which  case 
the  fabric  will  be  porous  to  air  but  will  shed  water  up  to  a  certain 
point. 

All  campers  are  familiar  with  the  unpleasant  fact  that  a  tent  will 
leak  in  a  rain  if  one  touches  the  canvas  with  one's  finger.  The 
tent  is  shower-proof  and  not  water-proof.  When  one  pushes 
against  the  tent,  it  either  contracts  or  expands  the  air  spaces. 
If  the  air  spaces  contract,  air  is  driven  out  and  when  the  finger  is 
taken  away,  water  enters.  If  the  air  spaces  expand,  water 
runs  in.  Either  way  the  air  is  displaced  at  that  point  and  water 
enters.  What  happens  can  best  be  seen  from  an  analogy.  If 
one  dips  a  fountain  pen  filler  into  ink  and  presses  the  bulb,  air 
is  driven  out  and  ink  enters  when  the  pressure  is  relieved.  If 
one  pinches  the  bulb  before  dipping  the  glass  end  into  the  ink, 
no  air  bubbles  out  but  ink  goes  in  as  soon  as  the  pressure  is  taken 
off.  It  makes  no  difference  therefore  whether  one  touches  a 
tent  from  the  inside  or  the  outside.  In  either  case,  water  is 
drawn  into  the  pores  of  the  fabric  and  the  tent  leaks  continuously 
once  the  air  is  displaced.  Awnings  on  a  boat  usually  leak  where 
they  rest  on  a  wooden  or  iron  support.  This  is  because  there  is 
always  enough  wind  or  vibration  to  be  equivalent  to  the  touching 
of  the  tent  with  the  finger.  For  the  same  reason,  a  tent  is  apt  to 
leak  around  the  pole. 

The  reverse  case  is  to  be  seen  in  the  ordinary  water  wings. 
They  are  made  of  a  fine-meshed  fabric  which  is  first  thoroughly 
wetted  so  that  a  film  of  water  is  formed  in  the  pores.  The 
strength  of  this  water  film  is  so  great  relatively  that  one  can  blow 
up  the  water  wings  without  the  air  escaping  and  the  water  wings 
will  support  a  grown  person  without  difficulty. 

Ehrenberg  and  Schultze2  point  out  that  many  powders,  when 
once  thoroughly  dried,  are  not  easy  to  wet.  After  a  period  of 
drought,  drops  of  rain  will  often  roll  along  the  dust  without  wet- 
ting it.  Even  in  the  case  of  a  shower  the  dust  may  be  only 
wetted  to  a  depth  of  less  than  a  quarter  of  an  inch.  This  is 

1  C.  O.  WEBER:  Jour.  Soc.  Dyers  and  Colourists,  17,  146  (1901). 

2  Zeit.  Kolloidchemie,  16,  183  (1914). 


72  APPLIED  COLLOID  CHEMISTRY 

shown  by  Ehrenberg  and  Schultze  to  be  due  to  the  adsorbed 
air  on  the  surface  of  the  solid.  Any  treatment  which  cuts  down 
the  amount  of  adsorbed  air  makes  the  dust,  or  soot,  or  other 
substance,  more  easily  wetted. 

It  is  not  easy  to  wet  carbon  black  with  water;  but  if  the  oily 
material  is  driven  out  by  a  preliminary  treatment  with  super- 
heated steam,  carbon  black  is  dispersed  by  water  very  rapidly 
and  the  liquid  bubbles  like  soda  water  owing  to  the  escape  of  the 
adsorbed  air.  If  a  50  cc  vessel  is  filled  with  good  cocoanut 
charcoal  and  if  water  is  then  added  until  the  vessel  is  full  up  to  the 
mark,  150-200  cc  of  air  will  be  driven  off. 

A  case  analogous  to  that  of  the  oiled  sieve  holding  water  has 
been  described  by  Lord  Rayleigh1  who  found  that  a  piece  of  iron 
gauze  pressed  down  upon  the  flat  bottom  of  a  glass  vessel,  holding 
a  shallow  layer  of  mercury,  remained  on  the  bottom  of  the  vessel 
and  did  not  rise  through  the  mercury.  It  is  quite  clear  from  this 
that  one  could  carry  mercury  in  an  iron  sieve.  With  sodium  in 
the  mercury  the  amalgam  would  run  through  the  gauze.  Ray- 
leigh 's  experiment  of  course  would  not  succeed  if  tried  with 
sodium  amalgam  instead  of  with  mercury. 

Since  the  adsorption  of  liquids  by  solids  is  selective,  it  follows 
that  one  liquid  will  be  adsorbed  more  by  a  solid  than  will  a 
second  liquid  and  that  consequently  the  first  will  displace  the 
second  from  contact  with  the  solid.  No  systematic  study  of  this 
phenomenon  seems  to  have  been  made;  but  we  know  that  kero- 
sene will  displace  water  in  contact  with  copper  and  that  water  will 
displace  kerosene  in  contact  with  quartz,2  while  alcohol  will 
displace  oil  in  contact  with  metal3  and  linseed  oil4  will  displace 
water  in  contact  with  white  lead.  According  to  this,  one  should 
use  a  rag  dipped  in  alcohol  with  which  to  wipe  off  a  lamp  and  it  is 
for  this  reason  that  we  cover  metal  surfaces  with  vaseline  or  with 
a  heavy  oil  to  prevent  rusting.  Parkerized  or  phosphatized  iron 
is  said  to  owe  its  non-rusting  properties  to  an  oil  film.  In  the 
kitchen,  people  wet  the  moulds  before  putting  in  corn  starch  or 
gelatine,  and  they  butter  the  pans  before  making  fudge,  or  grease 

1  Scientific  Papers,  4,  430  (1903). 

2  HOFMANN:  Zeifc.  phys.  Chem.,  83,  385  (1913). 
3PocKELs:  Wied.  Ann.,  67,  669  (1899). 

4  CRUICKSHANK  SMITH:  The  Manufacture  of  Paint,  92  (1915). 


ADSORPTION  OF  LIQUID  BY  SOLID  73 

the  griddles  before  making  flapjacks.  Jt  is  claimed  that  alumi- 
num griddles  do  not  need  to  be  greased  because  the  air  in  the 
oxide  film  keeps  the  cakes  from  sticking;  but  the  same  reasoning 
ought  to  apply  to  frying-pans  when  used  for  the  same  purpose 
and  apparently  does  not.  In  making  pottery,  it  is  the  practice 
to  oil  the  dies  when  making  dust-pressed  ware.1  Infusorial 
earth  is  excellent2  for  cleaning  glass  plates  for  photographic  pur- 
poses. Even  very  greasy  plates  become  clean  very  rapidly 
when  rubbed  with  infusorial  earth  moistened  with  water.  Some 
of  the  methods  of  removing  grease  spots  from  clothes  depend  on 
selective  adsorption.3 

There  are  almost  no  quantitative  data  on  selective  adsorption. 
Graham4  made  a  few  rough  measurements  on  the  selective  ad- 
sorption of  liquids  by  palladium.  A  piece  of  foil  was  immersed 
in  a  liquid  for  an  hour  and  afterwards  dried  by  pressure  for  a 
few  seconds  between  folds  of  blotting  paper.  One  thousand 
parts  of  palladium  foil  were  found  to  retain  1.18  of  water,  5.5 
of  alcohol,  1.7  of  ether,  0.54  of  acetone,  4.5  of  gylcerine,  3.5  of 
benzene,  10.2  of  castor  oil,  and  18.1  of  oil  of  sweet  almonds. 
Quite  recently  Mathers5  has  shown  that  lead  adsorbs  certain 
essential  oils  from  aqueous  solution. 

Since  the  adsorption  is  selective,  a  vapor  or  liquid  will  be  ad- 
sorbed more  readily  by  one  surface  than  by  another.  A  striking 
application  of  this  is  to  be  found  in  the  old  daguerreotype  proc- 
ess. The  latent  image  is  formed  on  a  plate  of  silver  coated  with 
silver  iodide  and  it  is  developed  by  means  of  mercury  vapor.  The 
mercury  condenses  on  the  places  which  have  been  exposed  to 
light.  I  have  seen  a  patched  cement  cellar  floor  on  which  mois- 
ture condensed  so  markedly  on  the  weathered  portion  that  it 
seemed  as  though  a  pipe  had  broken  under  the  floor. 

If  the  adsorption  is  very  marked  we  may  get  intense  strains.6 
Gelatine  is  adsorbed  so  strongly  by  glass  that  the  gelatine  film  on 
drying  will  often  tear  off  the  surface  of  the  glass.  On  the  other 

1  SIMCOE  and  SMITH:  Trans.  Am.  Ceramic.  Soc.,  14,  558,  567  (1912). 

2  ROLLER:  The  Utilization  of  Waste  Products,  310  (1915). 

3  LAKE:  Jour.  Phys.  Chem.,  20,  761  (1916). 

4  Jour.  Chem.  Soc.,  20,  275  (1867). 

5  Trans.  Am.  Electrochem.  Soc.,  31,  271  (1917). 
8  GRAHAM:  Jour.  Chem.  Soc.,  17,  320  (1864). 


74  APPLIED  COLLOID  CHEMISTRY 

hand,  it  does  not  adhere  to  plates  of  mica  or  calcite.  With  agar- 
agar  we  get  a  state  of  things  which  appears  to  vary  with  the 
concentrations.  Certain  agar-agar  jellies  will  adhere  readily  to 
glass  while  others  apparently  do  not.  People  have  claimed  that 
the  substitution  of  agar-agar  for  gelatine  in  photographic  plates 
would  be  impossible  on  a  technical  scale,  because  the  agar-agar 
film  would  slip  off  if  the  plates  were  set  vertically  to  dry;  but 
it  is  probable  that  this  difficulty  would  be  overcome  if  that  were 
the  only  drawback. 

The  whole  theory  of  adhesives  depends  in  part  on  the  fact  that 
the  cementing  material  adheres  strongly  to  the  two  surfaces  and 
hardens  there.  It  is  therefore  possible  that  one  agglutinant  may 
be  useful  for  a  number  of  different  materials,  such  as  wood,  glass, 
metal,  ivory,  etc.,  while  others  give  good  results  only  with  special 
materials.  Since  the  books  give  different  recipes  for  cements 
for  glass,  cements  for  metals,  cements  for  metals  and  glass,  etc., 
the  differences  in  adsorption  are  real  ones  though  no  one  has 
ever  made  a  careful  study  of  agglutinants  from  this  point  of  view. 

A  good  glue  joint  in  wood  will  easily  stand  a  strain  of  6000  Ibs. 
per  square  inch.  We  are  quite  familiar  with  this;  but  we  do  not 
always  realize  that  the  tensile  strength  of  liquid  water  is  pretty 
high  if  a  decrease  in  the  cross-section  can  be  prevented.  As  far 
back  as  1850  Berthelot1  filled  a  glass  tube  nearly  full  of  liquid, 
removed  the  air,  sealed  the  tube,  heated  the  tube  until  it  was 
entirely  full  of  liquid,  and  allowed  the  whole  thing  to  cool.  The 
liquid  continued  to  fill  the  tube  until  the  volume  was  larger  than 
it  should  be  for  the  temperature  by  1/420  in  the  case  of  water, 
1/93  in  the  case  of  alcohol,  and  1/50  in  the  case  of  ether.  This 
indicates  a  breaking  strain  of  about  50  atm.  for  water  and  over 
100  atm.  for  alcohol  and  ether.  Worthington2  obtained  17  atm. 
for  alcohol.  Stefan3  and  Tumlirz4  consider  that  the  calculated 
internal  pressure  of  a  liquid  is  a  measure  of  the  theoretical  tensile 
strength  and  they  obtain  in  this  way  a  value  for  alcohol5  of 
about  2000  atm.  Some  direct  measurements  of  the  tensile 

1  Ann.  Chim.  Phys.  (3)  30,  232  (1850). 

2  Phil.  Trans.,  183A,  355  (1892). 

3  Wied.  Ann.,  29,  655  (1896). 

«  Sitzungber.  Akad.  Wiss.  Wien.,  109  Ha,  837  (1900) ;  110 Ha,  437  (1901). 
6  Cf.  HULETT:  Zeit.  phys.  Chem.,  42,  353  (1903). 


ADSORPTION  OF  LIQUID  BY  SOLID  75 

strength  of  liquid  films  have  recently  been  made  by  Budgett,1 
who  found  a  maximum  breaking  strength  of  about  87  pounds  per 
square  inch  for  steel  end-gauges  held  together  by  a  water  film. 
On  the  assumption  that  the  water  occupied  only  one-tenth  of  the 
surface,  this  gives  a  tensile  strength  of  nearly  900  pounds  per 
square  inch,  or  about  sixty  atmospheres. 

Rice2  reports  that  certain  plastic  clays  from  Montana,  when 
worked  in  the  brick-making  machines  adhered  to  iron  in  a 
remarkable  way  whenever  they  came  into  contact  with  that  metal. 
Rice  does  not  explain  why  these  clays  should  stick  to  iron.  One 
of  the  clays  was  very  high  in  silica,  76.8  percent. 

The  formation  of  glazes  on  pottery  and  still  more  of  enamels  on 
metals  are  cases  in  which  the  fused  or  sintered  mass  must  not 
only  adhere  to  the  base  after  cooling  but  must  also  have 
nearly  enough  the  same  coefficient  of  expansion  to  prevent 
the  glass  or  enamel  from  going  to  pieces  as  the  temperature 
changes.3 

The  adsorption  of  water  vapor  by  gelatine  presents  some  pecu- 
liarities. Working  in  Ostwald's  laboratory,  von  Schroeder4 
found  that  gelatine  contained  a  thousand  percent  of  water  when 
immersed  in  liquid  water  and  less  than  400  percent  when  in 
saturated  water  vapor  at  the  same  temperature.  From  these 
and  other  experiments  von  Schroeder  concluded  that  the  vapor 
pressure  of  water  in  gelatine  must  be  higher  than  that  of  pure 
water,  because  water  distills  from  the  gelatine  to  the  vapor  phase. 
The  reason  for  this  unexpected  phenomenon  is  possibly  that 
gelatine  has  a  cellular  structure.  If  a  dry  paper  bag  be  placed  in 
a  space  saturated  with  water  vapor,  the  paper  will  adsorb  a 
certain  amount  of  water;  but  there  will  be  little  or  no  tendency 
for  the  water  to  condense  inside  the  bag  and  fill  it  with  liquid 
water.  If  a  similar  bag  is  immersed  in  liquid  water,  the  paper 
will  take  up  water  as  before;  but  water  will  also  pass  into  the  bag 
and  fill  it.  If  the  bag  is  then  lifted  into  the  vapor  phase,  and 
if  the  bag  is  so  constructed  that  the  actual  dripping  is  negligible, 
the  water  will  distill  from  the  curved  surface  in  the  bag  to  the 

» Proc.  Roy.  Soc.,  82A,  25  (1912). 

2  Trans.  Am.  Ceramic  Soc.,  14,  610  (1912). 

3  RANDAU:  Enamels  and  Enamelling,  101  (1912). 
«  Zeit.  phys.  Chem.,  46,  109  (1903). 


76  APPLIED  COLLOID  CHEMISTRY 

plane  surface  in  the  containing  vessel.  The  smaller  the  bag  is, 
the  greater  will  be  the  curvature  of  the  water  within  and  conse- 
quently the  higher  the  vapor  pressure.  With  the  microscopic 
spaces  in  solidified  gelatine,  the  vapor  pressure  of  the  drops  of 
water  will  be  enough  higher  to  counterbalance  the  difference  of 
levels  in  von  Schroeder's  experiments. 

While  this  explanation  accounts  for  the  phenomena  provided 
gelatine  really  has  a  cellular  structure,  a  point  which  is  not  yet 
generally  accepted — it  is  superfluous  if  the  facts  are  not  as  stated. 
Wolff  and  Buchner1  claim  that  the  loss  of  water  when  gelatine 
is  placed  in  the  vapor  phase  is  merely  due  to  experimental  error.2 
They  have  certainly  shown  the  possibility  of  error  in  von  Schroe- 
der's results;  but  they  did  no  experiments  involving  as  much 
error  as  one  must  assume  if  von  Schroeder's  data  are  worthless. 
The  whole  matter  is  therefore  up  in  the  air  for  the  present.  Wash- 
burn3  found  that  moistened  clay  will  dry  pretty  completely 
if  suspended  in  a  closed  vessel  above  water.  He  attributes  this 
to  the  action  of  gravity.  This  could  only  be  true  in  case  there 
were  no  soluble  salts  in  the  clay  and  in  case  the  pores  were  so 
coarse  that  the  capillary  rise  was  not  equal  to  the  distance 
above  the  water  level. 

There  are  other  substances  which  seem  to  behave  like  gelatine, 
though  no  experiments  like  von  Schroeder's  have  been  performed. 
Schwalbe4  states  that  cotton  cellulose  takes  up  21  percent 
moisture  in  presence  of  saturated  water  vapor.  Masson  and 
Richards5  find  17.7  percent  adsorbed  moisture  when  the  vapor 
phase  is  97  percent  saturated.  By  extrapolation  to  100  percent 
this  might  easily  give  about  21  percent  moisture;  but  Masson 
and  Richards  incline  to  the  view  that  the  curve  is  asymptotic, 
in  which  case  the  value  would  be  infinite,  which  seems  absurd. 
Vignon6  finds  that  one  gram  of  cotton  takes  up  about  five  grams 
of  liquid  water,  while  a  fine-grained  sponge  will  hold  thirty 
grams  of  water  per  gram  of  sponge,  and  a  coarse-grained  sponge 

1  Kon.  Akad.  Wet.  Amsterdam,  17,  May  30  (1914);  Zeit.  phys.  Chem.,  89, 
271  (1915), 

2  See  also  ARISZ:  Kolloidchemische  Beihefte,  7,  87  (1915). 

3  Jour.  Am.  Ceramic  Soc.,  1,  25  (1918). 

4  Die  Chemie  der  Cellulose,  10  (1911). 
5Proc.  Roy.  Soc.,  79A,  421  (1906). 

6  Comptes  rendus,  127,  73  (1898). 


ADSORPTION  OF  LIQUID  BY  SOLID  77 

only  about  six  grams.  Leighton1  states  that  absorbent  cotton 
will  hold  four  grams  of  water  when  centrifuged  at  4000  R.P.M. 
Some  of  this  water  is  held  in  capillary  spaces;  some  is  adsorbed  on 
the  surface  of  the  cotton;  a  part  is  probably  dissolved  in  the 
cotton;  and  some  may  be  held  in  cells.  We  have  no  satis- 
factory way  of  differentiating  between  these  possibilities. 

Dry  caseine  takes  up  about  1.25  g  water  per  gram  of  caseine.2 
Infusorial3  earth  is  able  to  take  up  more  than  double  its  weight  of 
liquid.  Wood  flour  will  take  up  over  double  its  weight  of  nitro- 
glycerine without  running.4  Fibrous  peat,  fresh  from  the  bog, 
may  contain  90  percent  of  water  and  Sphagnum  moss  may  im- 
bibe as  much  as  two  hundred  times  its  own  weight  of  water,5 
which  is  the  reason  that  this  moss  is  used  largely  in  the  trenches 
for  dressing  wounds. 

If  a  freshly-broken  piece  of  porous  earthenware  be  pressed 
against  the  tongue,  water  will  be  sucked  up  into  the  pores  of  the 
earthenware  and  the  piece  will  tend  to  stick  to  the  tongue. 
When  water  is  drawn  up  into  a  capillary  space,  air  is  driven  out 
and  one  at  once  wonders  how  much  pressure  the  water  will 
exert  under  these  circumstances.  Jamin6  has  made  some  pre- 
liminary experiments  along  this  line.  A  hole  was  bored  in  a 
piece  of  dried  chalk.  Into  this  hole  was  dipped  one  end  of  a 
manometer  and  the  hole  was  then  closed.  When  the  chalk  was 
placed  in  water,  the  air  was  displaced  from  the  pores  and  a  pres- 
sure of  3-4  atmospheres  was  obtained.  This  is  not  the  limiting 
pressure  because  the  amount  of  dead  space  in  the  manometer  was 
large.  A  better  method  would  be  to  determine  the  pressure 
necessary  for  the  air  to  force  the  water  out  of  the  pores  of  the  chalk. 

Spring7  obtained  pressures  of  one-fifth  of  an  atmosphere  when 
water  displaced  air  in  contact  with  sand. 

Jamin,  Askenasy,  and  Wiedeburg8  have  described  different 

1  Jour.  Phys.  Chem.,  20,  32  (1916). 

2  VAN   SLYKE  and   PUBLOW:  Science   and  Practice  of    Cheese- Making, 
179  (1913). 

3  ROLLER:  Utilization  of  Waste  Products,  310  (1915). 

4  KRESSMAN:  Met.  Chem.  Eng.,  14,  372  (1916). 

6  Percy's  Metallurgy:  Fuel,  208  (1875). 
'CHWOLSON:  Traite"  de  Physique,  1  III,  622  (1907). 

7  Mem.  Soc.  beige  Geologie,  97,  13  (1903). 

8  Wied.  Ann.,  69,  764  (1896). 


78  APPLIED  COLLOID  CHEMISTRY 

forms  of  apparatus  in  which  water  evaporates  through  a  porous 
tube  or  a  porous  cup  of  plaster  into  dry  air.  As  the  water  evapor- 
ates off,  mercury  is  drawn  up  into  the  tube  reaching  a  height  of  90 
cm  without  difficulty.  If  bubbles  of  air  did  not  form,  water  or 
mercury  could  be  drawn  up  to  incredible  heights,  limited  by  the 
tensile  strength  of  the  liquids. 

When  considering  the  adsorption  of  water  vapor  by  cotton, 
reference  was  made  to  the  heat  developed.  •  Attention  might 
perhaps  be  called  here  to  Meissner's  experiments  on  the  heat 
developed  when  powders  are  wetted  by  liquids.1  He  found  that 
there  was  no  appreciable  difference  in  the  heat  evolved  when 
water  was  taken  above  and  below  4°.  His  quantitative  results 
were  rather  discouraging,  however,  because  he  found  no 
apparent  relation  between  the  amounts  of  heat  evolved  and  the 
amounts  of  powder  taken,  which  must  be  the  result  of  experi- 
mental error.  The  more  careful  experiments  of  Parks2  showed 
that  when  silica,  sand  or  glass  is  brought  in  contact  with  water 
at  approximately  constant  temperature,  the  development  of 
heat  at  7°  is  approximately  0.00105  cal.  per  square  centimeter  of 
surface  wetted. 

Foote  and  Saxton3  find  that  water  adsorbed  by  lamp-black 
supercools  readily  to  —  6°  and  is  not  all  frozen  at  —  35°.  When 
once  frozen,  the  water  does  not  begin  to  melt  until  nearly  0° 
in  the  case  of  lamp-black.  Water  adsorbed  in  gelatinous  pre- 
cipitates may  start  melting  at  —  20°.  Mclntosh  and  Edson4  find 
that  when  a  salt  solution  is  plunged  into  liquid  air  or  solid  carbon 
dioxide,  the  resulting  solid  forms  a  constant-melting  mixture, 
melting  completely  at  the  temperature  at  which  the  solution  should 
have  started  freezing.  It  is  not  clear  why  this  should  be  so. 

ADSORPTION  OF  SOLID  BY  SOLID 

Dried  glue  may  be  considered  either  as  a  solid  or  as  a  super- 
cooled liquid.  If  we  consider  it  as  a  solid,  then  the  adsorption 
of  gelatine  by  glass  is  a  case  of  adsorption  of  a  solid  by  a  solid. 
It  is  more  satisfactory  to  have  cases  where  there  can  be  no  doubt 

IMEISSNER:  Wied.  Ann.,  29,  114  (1886). 

2  Phil.  Mag.  (6)  4,  247  (1902). 

3  Jour.  Am.  Chem.  Soc.,  38,  588  (1916);  39,  627  (1917). 

4  Ibid.,  38,  613  (1916). 


ADSORPTION  OF  LIQUID  BY  SOLID  79 

but  that  both  substances  are  solids.  We  get  marked  adsorption 
of  rouge  by  metals.1  If  a  metal  is  polished  with  rouge  which  is 
not  kept  sufficiently  moist,  the  rouge  adheres  so  firmly  to  the 
metal  surfaces  that  it  cannot  be  removed  without  spoiling  the 
polish.  Blue  powder  is  another  case  of  adsorption.  Here  we 
have  grains  of  zinc  coated  with  zinc  oxide  which  adheres  even 
when  the  zinc  is  melted.  Aluminum  and  nickel  are  always 
coated  with  solid  surface  films  which  have  marked  effects  on  the 
properties  of  the  metals.  Passive  iron  owes  its  peculiar  proper- 
ties to  the  adsorption  and  stabilization  of  a  higher  oxide,2 
either  FeC>2  or  FeOa.  It  seems  probable  that  vermilion  is 
adsorbed  by  copper  because  De  la  Rue3  says  that  electroplated 
copper  blocks  cause  vermilion  to  blacken  while  cast  copper  does 
not. 

We  get  what  seems  to  be  a  case  of  adsorption  of  a  solid  by  a 
solid  if  we  shake  a  moderately  coarse  powder  with  a  much  finer 
powder.  Instead  of  the  finer  powder  filling  the  voids  in  the  space 
occupied  by  the  coarse  powder,  the  former  tends  to  coat  the  lat- 
ter.4 If  the  two  powders  are  of  different  colors,  the  color  of  the 
coarser  powder  may  be  masked  completely  by  a  relatively  small 
amount  of  the  fine  powder.  In  one  experiment  0.032  g  of  Prus- 
sian blue  was  mixed  with  10  g  dolomite  which  passed  a  40  mesh 
sieve  and  did  not  pass  a  100  mesh  sieve.  In  the  second  experi- 
ment the  same  amount  of  Prussian  blue  was  mixed  with  10  g 
dolomite,  all  of  which  would  pass  a  200  mesh  sieve.  In  the 
the  first  case,  the  powder  was  a  deep  blue  and  in  the  second  it 
was  practically  white,  though  the  percentage  compositions  were 
the  same  in  the  two  cases.  In  another  experiment  1  percent 
Fe-jOa  colored  the  dolomite  a  full  red  when  the  rouge  passed  a 
200  mesh  sieve  and  the  dolomite  did  not  pass  a  100  mesh  sieve. 
When  the  dolomite  passed  a  200  mesh  sieve  and  the  iron  oxide 
did  not  pass  a  100  mesh  sieve,  the  powder  was  practically  white. 
In  a  third  set  of  experiments  8  milligrams  of  lamp-black  black- 
ened 10  g  dolomite  which  was  coarser  than  40  mesh.  When  the 
dolomite  was  ground  so  that  it  all  passed  a  200  mesh  sieve,  654 

1  DESCH:  Metallographie,  210  (1914). 

2  BENNETT  and  BURNHAM:  Jour.  Phys.  Chem.,  21,  107  (1917). 

3  Mem.  Chem.  Soc.,  2,  305  (1845). 

«  FINK:  Jour.  Phys.  Chem.,  21,  32  (1917);  BRIGGS:  Ibid.,  22,  216  (1918). 


80  APPLIED  COLLOID  CHEMISTRY 

milligrams  of  lamp-black,  or  over  eighty  times  as  much,  were 
needed  to  produce  approximately  the  same  tint. 

This  behavior  of  powders  is  of  great  importance  when  using 
inert  fillers  in  mixed  paints.1  The  amount  of  barytes  that  can 
be  mixed  with  colored  pigments  without  injuring  them  is  re- 
markably large.  There  are  hundreds  of  brands  of  para-red 
paints  made  and  consumed  every  year  by  the  agricultural  imple- 
ment trade  which  contain  as  high  as  90  percent  of  natural 
barytes.  The  opacity  of  paper  may  be  increased  by  grinding 
the  filler  more  finely. 

Fink  has  shown  that  a  given  mixture  of  a  white  non-conducting 
powder  like  thoria  with  a  black  conducting  powder  like  tungsten 
may  either  be  black  and  conduct  electricity  or  white  and  a  non- 
conductor, depending  on  the  relative  coarseness  of  the  two  pow- 
ders. If  one  strews  lycopodium  powder  over  the  surface  of  water 
and  then  puts  one's  finger  into  the  water,  the  lycopodium  powder 
sticks  to  the  finger  and  keeps  it  from  being  wetted.2  A  coarse 
powder  kept  in  a  bottle  will  fall  to  the  bottom  of  the  bottle 
through  the  force  of  gravity  while  a  very  fine  powder  will  stick 
to  the  walls  of  the  bottle  discoloring  them.  Writing  on  paper 
with  a  pencil  or  on  a  blackboard  with  chalk  is  a  phenomenon 
of  about  the  same  type,  though  the  matter  is  simplified  by  the 
roughened  surface  on  which  one  writes.  Solid  particles  are  rub- 
bed off  and  stick.  It  seems  probable  that  the  possibility  of 
using  face  powder  is  merely  a  special  case  of  a  very  fine  powder 
adhering  to  a  curved  surface. 

In  phosphate  baking  powders,  the  monocalcium  phosphate  is 
protected  from  moisture  by  an  admixture  of  starch  and  it  is 
well-known  in  the  trade  that  the  calcium  phosphate  must  not 
be  too  fine  nor  the  starch  too  coarse;  potato  starch  cannot  be  sub- 
stituted for  corn  starch.  The  retarding  effect  on  the  rate  of 
setting  of  Portland  cement,  which  is  produced  by  the  addition 
of  gypsum  is  probably  due  to  the  fact  that  the  finer  gypsum  powder 
coats  the  coarser  cement  powder  and  acts  to  some  extent  as  a 
protecting  film. 

In  all  these  cases  the  powders  do  not  come  in  actual  contact, 
but  are  separated  by  a  film  of  air  which,  however,  holds  them 

1  TOCH:  The  Chemistry  and  Technology  of  Paints,  113,  138  (1916). 

2  EHRENBERG  and  SCHULTZE:  Zeit.  Kolloidchemie,  16,  183  (1914). 


ADSORPTION  OF  LIQUID  BY  SOLID  81 

together,  thus  behaving  like  a  liquid  film.  Traces  of  moisture 
increase  the  adhesion,  as  in  the  case  of  sugar  and  blueberries. 
The  fine  particles  will  be  held  more  firmly  in  hollows  in  the  coarser 
powders  than  on  projections  and  will  therefore  accumulate  first 
in  the  concave  portions,  next  on  the  plane  surfaces,  and  last  of 
all  on  the  convex  surfaces.  Aitken1  found  that  hot  powders 
adhered  to  a  cold  rod  of  glass  or  metal;  but  fell  off  when  powder 
and  rod  were  of  the  same  temperature  and  were  repelled  when 
the  rod  was  hotter  than  the  powder.  This  is  due  to  a  lowering  of 
the  surface  tension  of  the  condensed  air  film  on  the  hotter  side 
of  the  powder.  It  is  necessary  to  work  with  such  coarse  particles 
that  they  fall  off  under  the  influence  of  gravity  except  when  special 
conditions  prevail. 

It  has  been  shown  that  the  time  factor  is  an  important  one  in 
the  wetting  of  some  solids  by  some  liquids,  the  air  film  being 
displaced  slowly.  It  is  possible  that  in  some  cases  adhering 
powders  displace  the  air  film  slowly  and  become  really  adsorbed 
by  the  supporting  solid. 

ADSORPTION  OF  LIQUID  BY  LIQUID 

The  spreading  of  one  liquid  over  another  is  usually  considered 
to  depend  on  the  relative  surface  tensions.     If  oil  is  dropped  on 
water,  it  is  assumed  that  we  have  a  water-air  surface,  a  water- 
oil  surface,  and  an  oil-air  surface  as 
shown  in  Fig.  4.     If  the  surface  ten- 

/4//T* 

sion  water-air  is  greater  than  the  sum 

of  the  surface  tensions  oil-water  and 

oil-air,   the   drop  of   oil  spreads  out 

over  the  surface  of  the  water.     If  the 

water-air  surface  tension  is  less  than 

the  sum   of  the   other   two  the  oil  FIG.  4. 

will  draw  up  into  a  globule.     While 

this  may  be  a  perfectly  correct  way  of  looking  at  things,  it  is 

not  a  useful  one  because  Rayleigh2  claims  that  three  fluids  cannot 

be  in  contact  simultaneously.     The  simplest  way  to  look  at  the 

1  Trans.  Roy.  Soc.  Edin.,  32,  239  (1884);  TAMMANN:  Drude's  Ann.,  18, 
856  (1905). 

2  Scientific  Papers,  3,  354,  413,  416  (1902). 


82  APPLIED  COLLOID  CHEMISTRY 

matter  is  to  consider  the  adsorption.  If  oil  is  adsorbed  by  water, 
it  wets  the  water  and  spreads  over  the  surface.  Now  the  ad- 
sorbed layer  may  be  very  thin,  not  over  2/x/x  in  some  cases.  At 
greater  thicknesses  there  is  nothing  to  prevent  the  oil  tearing 
loose  from  the  oiled  surface  of  the  water  and  drawing  up  into 
drops  under  the  influence  of  surface  tension.  This  has  been 
commented  on  by  Rayleigh1  and  by  Budgett.2 

The  first  portion  of  oil  spreads  over  the  water  because  it  is  ad- 
sorbed and  held  by  the  water.  The  second  portion  of  oil  flows 
out  over  the  oiled  surface,  but  is  instable  because  the  same  mass 
of  oil  would  have  a  less  surface  if  present  in  drops  and  still  more 
so  if  present  in  a  single  drop.  The  excess  oil  therefore  draws 
up  into  one  or  more  drops  depending  on  conditions,  thereby 
leaving  bare  oiled  surfaces.  The  several  drops  will  coalesce 
into  one  large  one  if  brought  in  contact.  If  more  oil  is  added 
the  drops  become  larger  and  finally  form  so  large  a  drop  that  under 
the  influence  of  gravity,  it  covers  the  whole  surface  of  the  water 
and  becomes  a  liquid  layer  of  free  oil.  From  this  way  of  looking 
at  it,  we  see  that  the  phenomenon  must  be  general  and  so  it  is. 
Budgett3  found  that  it  was  quite  impossible  to  get  a  thin  continu- 
ous film  of  kerosene  or  water  on  the  surface  of  steel  blocks. 
Microscopic  examination  showed  that  the  applied  films  were 
not  continuous  membranes  but  that  the  liquid  collected  into 
small  drops.-  This  is  undoubtedly  exactly  like  Rayleigh's  ex- 
periment. On  the  surface  of  the  wetted  steel  the  kerosene  or 
water  gathered  in  small  drops.  With  different  liquids,  we  should 
expect  to  find  the  drops  first  forming  when  the  films  reached 
different  thicknesses.  This  has  been  noted  by  Hardy4  who  found 
that  the  thickness  of  the  oil  film  varied  from  2-200^^  with  dif- 
ferent oils.  With  one  sample  of  oil  there  was  apparently  no 
spreading  at  all. 

If  an  oil  is  not  adsorbed  by  water,  there  is  theoretically  a  film 
of  air  between  the  water  and  the  drop  of  oil.  It  seems  improbable 
that  this  air  film  can  continue  to  exist  when  a  thick  layer  of  oil 
is  present  and  yet,  if  not,  there  must  be  some  point  under  some 

1  Scientific  Papers,  3,  424  (1902). 

2  Proc.  Roy.  Soc.,  85A,  30  (1911). 

3  Ibid.,  86A,  30  (1911). 

4  Ibid.,  86A,  612  (1912). 


ADSORPTION  OF  LIQUID  BY  SOLID  83 

conditions  when  surfaces  of  oil,  water,  and  air  coexist.  It  seems 
probable  that  Rayleigh's  conclusion,  that  such  a  state  of  things 
cannot  exist,  holds  only  when  the  effect  of  gravity  is  ignored. 
With  increasing  amount  of  oil  there  probably  comes  a  time  when 
the  air  film  is  squeezed  out  entirely.  The  breaking  up  of  a  liquid 
film  into  drops  has  been  paralleled  by  the  behavior  of  thin  solid 
1  films  when  heated.  Beilby1  showed  that  a  very  thin  film  of  gold 
leaf  became  granular  if  heated  moderately. 

ADSORPTION  OF  SOLID  BY  LIQUID 

A  solid  which  is  wetted  by  a  liquid,  adheres  to  that  liquid. 
With  two  liquids  and  one  solid  we  get  selective  adsorption  of  the 
solid  and  this  leads  to  some  interesting  results,  which  have  been 
studied  carefully  by  Hofmann.2  If  finely  divided  red  lead  is 
shaken  with  water  and  benzene  or  chloroform,  the  red  lead  ad- 
heres to  the  surface  of  the  benzene  or  chloroform,  making  a 
practically  continuous  coating  around  the  organic  liquid  pro- 
vided proper  relative  amounts  of  red  lead  and  organic  liquid  are 
taken.  The  theory  of  the  phenomenon  has  been  given  byDes 
Coudres.3  The  solid  particles  tend  to  go  into  the  water  phase  if 
they  adsorb  water  to  the  practical  exclusion  of  the  other  liquid; 
they  tend  to  go  into  the  other  liquid  phase  if  they  tend  to  adsorb 
the  other  liquid  to  the  practical  exclusion  of  the  water;  while 
the  particles  tend  to  go  into  the  dineric  interface  in  case  the 
adsorption  of  the  two  liquids  is  sufficiently  intense  to  increase 
the  miscibihty  of  the  two  liquids  very  considerably  at  the  sur- 
face between  solid  and  liquid. 

Of  course  the  powder  will  only  stay  in  the  upper  phase  or  the 
dineric  interface  in  case  it  is  so  fine  that  the  adsorption  over- 
balances gravity.  These  conclusions  as  to  the  way  a  given  pair 
of  liquids  will  behave  with  a  given  solid  refer  to  the  relations  pre- 
vailing under  the  conditions  of  the  experiment.  Hofmann's 
experiments  with  two  liquid  layers  in  glass  test-tubes  (soda-lime 
glass)  showed  that  the  time  factor  might  be  an  important  one. 
If  a  few  cubic  centimeters  of  an  aqueous  potassium  bichromate 

1  Proc.  Roy.  Soc.,  72,  226  (1904). 

2  Zeit.  phys.  Chem.,  83,  385  (1913). 

3  Arch.  Entwicklungsmechanik,  7,  325  (1898). 


84  APPLIED  COLLOID  CHEMISTRY 

solution  are  shaken  up  in  a  carefully  cleaned  glass  test-tube,  there 
remains  on  the  walls  of  the  test-tube  a  continuous  uniform  coat- 
ing of  solution  recognizable  by  its  yellow  color.  This  film  gradu- 
ally becomes  thinner  and  paler  but  does  not  break  at  any  point 
until  the  water  disappears  completely  through  evaporation. 
If  one  adds  a  layer  of  xylene  or  kerosene  to  the  aqueous  bichro- 
mate solution  in  the  test-tube,  it  is  easy  to  see  that  there  is  a  coating 
of  aqueous  solution  between  the  organic  liquid  and  the  glass.  If 
water  is  allowed  to  flow  in  at  the  bottom  of  the  test-tube,  the  or- 
ganic liquid  can  be  forced  out  at  the  top  and  there  will  be  no 
sign  of  its  having  adhered  to  the  glass  wall  at  any  point.  If, 
however,  the  test-tube  containing  the  aqueous  bichromate  solu- 
tion and  the  xylene  or  kerosene  be  left  standing  quietly  for  a 
long  time,  6-10  hours,  it  will  be  found  that  the  organic  liquid 
has  diffused  through  the  water  film  to  some  extent  and  now  ad- 
heres to  the  glass  wall  nearly  everywhere,  having  displaced  the 
aqueous  film  almost  completely.  If  the  contents  of  the  test-tube 
are  shaken  violently,  the  water  displaces  the  organic  liquid  from 
the  glass  surface  practically  completely.  The  glass  is  therefore 
wetted  more  readily  and  more  rapidly  by  water  than  by  xylene 
or  petroleum;  but  a  large  mass  of  xylene  or  petroleum  will  gradu- 
ally displace  a  water  film  from  the  glass  surface.  One  would 
therefore  expect  to  get  a  slightly  different  behavior  with  glass 
powder  according  as  one  treated  it  first  wi th  water  01  with  organic 
liquid.  This  was  exactly  what  Hofmann  found.  If  the  glass 
powder  were  treated  first  with  water  and  then  shaken  with  kero- 
sene, practically  all  the  glass  went  into  the  water  phase.  If  the 
glass  powder  were  allowed  to  stand  a  long  time  with  kerosene,  or 
if  it  were  heated  with  it  to  hasten  the  removal  of  the  air  film,  some 
of  the  glass  powder  would  remain  in  the  dineric  interface.  Vig- 
orous shaking  would,  of  course,  cause  the  water  to  displace  the 
kerosene.  Chloroform  behaves  towards  glass  and  water  some- 
what like  xylene  or  kerosene  though  the  tendency  to  displace 
the  water  layer  from  the  glass  is  less  than  with  kerosene.  Small 
drops  of  chloroform  do  not  break  through  the  water  film  at  all. 
With  increasing  amounts  of  chloroform  the  force  of  gravity  causes 
it  to  break  through  at  the  bottom  of  the,  test-tube,  the  surface 
wetted  by  the  chloroform  increasing  as  the  column  of  chloroform 
increases  in  height. 


ADSORPTION  OF  LIQUID  BY  SOLID 


85 


In  Table  XIX  are  given  some  of  Hofmann's  results.  The 
letter  w  means  that  the  solid  goes  nearly  completely  into  the 
water  phase;  while  o  denotes  that  it  goes  almost  completely  into 
the  organic  liquid;  s  means  that  the  powder  collects  nearly  quanti- 
tatively in  the  interface.  Two  letters,  such  as  sw,  mean  that 
the  powder  goes  partly  into  the  interface  and  somewhat  less  into 
the  water  phase;  the  use  of  a  parenthesis,  s(w)  or  s(o\  signifies 
that  there  is  a  good  deal  more  powder  in  the  interface  than  in  the 
bracketed  phase.  Benzene  and  xylene  behave  alike  so  only  one  is 
given.  In  the  experiments  tabulated,  the  powder  was  placed 
in  water  and  then  shaken  with  the  organic  liquid. 


TABLE  XIX. — PREFERENTIAL  WETTING  OF  SOLIDS 


Water  and 

Ether 

Chloro- 
form 

Butyl 
alco- 
hol 

Benzene 

Kero- 
sene 

Amyl 
alco- 
hol 

Paraffin 
oil 

CaSO4 

W 

w 

W 

W 

W 

W 

W 

SnO2  
Al(OH),  
SnS  

«.•(«) 
w(s) 
ws 

ws 
ws 
ws 

WS 

ws 
ws 

s(w) 
s 
s(w) 

s(w) 

8(wf) 

s(w) 

sw- 
sw 

g 

SW 

s(w) 

WS 

BaSO4  

ws 

ws 

ws 

s 

g 

s(w^) 

s(w?) 

ZnS 

w(s) 

ws 

ws 

s 

s(wtf) 

g 

0(711?') 

ZnO  

ws 

ws 

s(w) 

s 

g 

g 

§ 

CaCO3 

WS    i 

ws 

s 

s 

g 

g 

§ 

Mg(OH)2  

s(wl} 

ws 

s 

s 

g 

s(w) 

sw 

Al  

sw 

g(w) 

s(w) 

s 

g 

s(w) 

g 

BaCO3 

ws 

ws 

s 

s 

g 

sw 

g 

CuS  

ws 

s(w) 

s 

s 

g 

g 

g 

PbCrO4..    .    . 

ws 

s(wf) 

s 

s 

g 

g 

g 

Cu2O(?) 

ws 

s 

s 

s 

g 

g 

g 

MoS2  
PbS  

8(W?) 

ws 

s 

s 
s 

s 
s 

s 
s 

s 

s 

g 

Fe3O4 

ws 

s 

s 

§ 

g 

g 

BaCrO4  

ws 

s 

s 

s 

g 

g 

g 

Pb3O4  

sw 

s 

s 

s 

g 

g 

c 

sw 

s 

§ 

s 

g 

g 

g 

PbI2  

s 

s 

OS 

s 

g 

OS 

g 

HgS  . 

s 

s 

s 

s 

g 

g 

g 

HeO 

s 

s 

s 

g 

g 

g 

g 

HeI2 

s 

g 

s 

g 

Agl.  . 

s 

s 

Q 

g 

Q 

OS 

86 


APPLIED  COLLOID  CHEMISTRY 


Much  the  same  conclusions  have  been  worked  out  by  Reinders1 
independently  of  Hofmann.  Some  of  his  data  are  given  in 
Table  XX.  The  powder  was  placed  in  the  water  phase  and  then 
shaken  with  the  organic  liquid. 

TABLE  XX. — PREFERENTIAL  WETTING  OF  SOLIDS 


Water  and 

Paraffin 
oil 

Amyl 
alcohol 

ecu 

Benzene 

Ether 

Kaolin  

w 

w(s) 

w(s) 

W 

w(s) 

CaF2 

ws 

WS 

w(s) 

w(s) 

w(s) 

Gypsum  

w 

ws 

w 

SW 

WS 

BaSO4  

w(s) 

ws 

ws 

sw- 

WS 

Magnesium 

ws 

ws 

ws 

ws 

ws 

PbO  
Malachite.           

s 
so 

s 
s 

sw 
s 

s 

sw 
sw 

ZnS  .    . 

s 

s 

s 

s 

sw 

PbS  
HffI2  . 

so 
so 

so 
s 

s 
s 

s 
s 

s 
s 

Carbon  

so 

s 

s 

s 

Selenium  
Sulphur 

so 
so 

so 
so 

so 
o(s) 

s 
so 

s 

Similar  results  were  obtained  with  colloidal  solutions,  Isobutyl 
alcohol  was  added  to  a  colloidal  gold  solution  obtained  by  reduc- 
ing gold  chloride  with  carbon  monoxide.  When  the  two  liquids 
are  shaken,  the  gold  forms  a  thin  film  at  the  interface.  This  film 
is  violet  blue  to  blue  green  by  transmitted  light  and  golden  by 
reflected  light.  A  thin  water  film  forms  between  the  isobutyl 
alcohol  and  the  glass,  and  the  gold  concentrates  in  the  dineric 
interface  thus  formed,  making  the  alcohol  appear  uniformly 
gold-plated.  With  ether  the  gold  film  rises  high  above  the  level 
of  the  two  liquids.  With  carbon  bisulphide  the  adherent  film  of 
gold  appears  blue.  When  the  carbon  bisulphide  is  broken  into 
drops  by  shaking,  each  drop  appears  blue.  When  a  blue  gold 
was  obtained  by  reducing  gold  chloride  with  phosphorus  dissolved 
in  ether,  the  gold  went  into  the  dineric  interface.  When  a  brown- 
ish red  gold  was  obtained  in  this  way,  it  remained  in  the  water 
phase  and  showed  no  tendency  to  pass  into  the  interface.  This 
difference  is  undoubtedly  due  to  an  adsorption  of  something  at  the 

1  Zeit.  Kolloidchemie,  13,  235  (1913). 


ADSORPTION  OF  LIQUID  BY  SOLID  87 

surface  of  the  gold  because  Reinders  found  that  0.005  percent 
gum  arable  prevents  colloidal  gold  from  passing  into  the  ether- 
water  interface.  With  carbon  tetrachloride,  carbon  bisulphide, 
and  benzene,  the  gold  goes  to  the  interface  as  before;  but  the 
gum  arabic  prevents  its  changing  from  red  to  blue. 

With  colloidal  ferric  oxide  practically  no  iron  went  into  the 
interface  with  any  of  the  organic  liquids.  It  seems  probable 
that  this  is  due  to  Reinders  using  a  ferric  oxide  solution  which  was 
a  year  and  a  half  old,  because  hydrous  ferric  oxide  is  ordinarily 
a  pretty  good  emulsifying  agent.1  Colloidal  arsenic  sulphide 
goes  into  the  dineric  interface  with  amyl  alcohol  or  butyl  alcohol ; 
but  stays  in  the  water  phase  when  carbon  tetrachloride,  benzene, 
or  ether  is  the  second  liquid.  India  ink  goes  completely  into 
the  interface  with  isobutyl  alcohol  and  stays  entirely  in  the  water 
phase  when  ether  is  the  second  liquid.  When  crystallized  copper 
eosinate  is  placed  in  water  and  shaken  with  ether,  it  goes  entirely 
into  the  surface  between  the  two  layers,2  giving  a  film  which 
resembles  the  purple  skin  of  a  grape,  by  reason  of  the  steel-blue 
color  of  the  crystals. 

An  interesting  experiment  is  to  shake  copper  powder  or  alumi- 
num powder  with  kerosene  and  water.  The  metallic  powder 
goes  into  the  kerosene  and  into  the  interface,  producing  an  effect 
of  molten  copper  or  molten  aluminum  as  the  case  may  be.  When 
the  bottle  is  allowed  to  stand  after  having  been  shaken,  the 
metallic  powder  in  the  interface  creeps  up  the  side  of  the  bottle 
above  the  surface  of  the  liquid,  rising  higher  if  a  little  alcohol  has 
been  added.  I  have  seen  an  apparently  coherent  metallic  film 
rise  two  or  three  inches  above  the  surface  of  the  upper  liquid 
phase.  If  too  much  copper  or  aluminum  be  added  the  kerosene 
cannot  hold  it  all  up  and  a  portion  falls  to  the  bottom  of  the  flask 
carrying  drops  of  kerosene  with  it.  If  the  mixture  be  poured 
out  on  a  piece  of  wood,  the  copper  spreads  over  the  surface  of  the 
wood  just  as  it  did  over  the  surface  of  the  glass.  This  experi- 
ment illustrates  the  principle  involved  in  all  bronzing  liquids. 
A  bronzing  liquid  consists  of  a  volatile  liquid  which  will  hold  up 
the  metal,  and  some  substance  which  will  keep  the  metallic 
powder  from  rubbing  off  too  readily  after  it  has  been  applied. 

1  BRIGGS:  Jour.  Phys.  Chem.,  19,  296  (1915). 

2  GILBERT:  Jour.  Phys.  Chem.,  18,  602  (1914). 


88  APPLIED  COLLOID  CHEMISTRY 

The  second  substance  also  serves  the  purpose  of  keeping  the 
metallic  powder  in  a  better  state  of  suspension  in  the  volatile 
liquid.  According  to  Worden1  the  best  of  these  bronzing  liquids 
consist  essentially  of  pyroxylin  dissolved  in  amyl  acetate,  to 
which  the  metallic  powder  is  added.  For  bronzing  radiators 
and  steam  pipes,  a  pure  pyroxylin  would  not  do  since  it  would 
decompose,  disintegrate,  and  allow  the  metallic  coating  to  peel 
off.  To  prevent  this,  boiled  linseed  oil  and  rosin  may  be  added. 
Such  preparations  are  of  course  much  slower  in  drying  than  pyroxy- 
lin bronzing  liquids;  but  that  is  not  a  serious  objection  for 
these  special  purposes.  A  much  cheaper  form  of  bronzing 
liquid  consists  of  rosin  dissolved  in  benzene,  while  the  cheapest 
of  all  is  apparently  a  solution  of  sodium  silicate  in  water.  The 
aluminum  and  copper  powders  on  the  market  are  coated  with 
stearin;  but  special  experiments  in  my  laboratory  have  shown  that 
the  behavior  of  copper  and  aluminum  with  kerosene  is  the  same 
qualitatively  whether  the  stearin  coating  is  removed  with  ether 
or  not. 

Barus2  and  Rayleigh3  have  each  noticed  that  when  fine  powders 
(bole)  or  dust  are  shaken  with  ether  and  water  the  solid  particles 
go  into  the  water  layer,  leaving  the  ether  layer  astonishingly 
clear.  In  fact  Rayleigh  said  that  the  ether  layer  was  more 
free  of  motes  than  he  had  ever  been  able  to  obtain  by  repeated 
distillation  of  liquid  in  vacuum.  White  lead  passes  from  water 
into  linseed  oil.4  " During  recent  years  the  practice  has  been 
adopted  largely  among  white  lead  corroders  who  grind  their 
own  white  lead  in  oil  of  doing  away  with  the  final  drying  of  the 
white  lead  pulp  as  it  comes  from  the  washing  process,  and  grind- 
ing or  beating  up  the  pulp  (exhausted  of  water  until  the  propor- 
tion of  the  latter  does  not  exceed  about  20  percent)  with  a 
suitable  quantity  of  refined  linseed  oil.  This  process  depends 
on  the  greater  surface  attraction  which  white  lead  particles  offer 
to  linseed  oil  than  to  water.  It  enables  considerable  economies 
to  be  effected  in  the  manufacture  of  'ground  white  lead'  and  it 
eliminates  risk  of  lead  poisoning  during  one  of  the  most  dangerous 

1  Nitrocellulose  Industry,  1,  310-320  (1911). 

2  Am.  Jour.  Sci.  (3)  37,  124  (1889). 

3  Scientific  Papers,  3,  569  (1902). 

4  J.  CRUICKSHANK  SMITH:  The  Manufacture  of  Paint,  92  (1915). 


ADSORPTION  OF  LIQUID  BY  SOLID  89 

parts  of  the  white  lead  manufacturing  process."  Zinc  oxide 
stays  in  the  water  phase.  The  Elmore  bulk-oil  process  of  ore 
flotation  depended  on  the  fact  that  some  sulphide  ores  can  be 
shaken  out  of  the  water  phase  by  means  of  suitable  oils  while  the 
gangue  stays  in  the  water  phase.  When  ground  bituminous 
coal  is  stirred  up  with  fuel  oil  and  water,  there  is  a  good  separation 
of  coal  from  ash,  the  former  going  into  the  oil  and  .the  latter 
into  the  water.  The  readiness  with  which  solid  films  form  over 
the  surface  of  mercury  is  well  known  to  everybody.  Salts  of 
mercury  or  of  the  baser  metals  coat  mercury  with  great  readiness, 
especially  in  the  air. 


CHAPTER  III 
ADSORPTION  FROM  SOLUTION 

ADSORPTION  FROM  SOLUTION  BY  SOLID 

Selective  adsorption  by  a  solid  from  a  solution  may  show  itself 
by  relatively  more  of  the  dissolved  substance  being  adsorbed 
than  of  the  solvent,  or  by  relatively  less  being  adsorbed.  In  the 
first  case,  commonly  called  positive  adsorption,  the  solution 
becomes  more  dilute  and  in  the  second  case,  commonly  called 
negative  adsorption,  the  solution  becomes  more  concentrated. 
Both  cases  are  known;  but  the  first  one  is  the  more  important 
and  will  be  considered  now.  The  decolorizing  action  of  char- 
coal1 has  been  known  since  1791.  In  1874  Bottger2  pointed  out 
that  when  an  alcoholic  solution  of  any  of  the  aniline  dyes  is  shaken 
up  with  a  sufficient  quantity  of  infusorial  earth,  some  water 
added,  and  the  whole  thrown  on  a  paper  filter,  the  liquid  which 
runs  through  is  perfectly  colorless,  the  pigment  being  retained  by 
the  siliceous  earth.  The  adsorption  of  dyes  by  hydrous  alumina, 
stannic  oxide,  and  other  mordants  as  they  are  called,  is  of  great 
importance  in  dyeing.  Here  as  in  all  other  cases  the  adsorption 
is  selective.  Da vison3  showed  thatwhen  alumina  is  precipitated 
from  an  aluminum  acetate  solution,  it  will  decolorize  dilute 
solutions  of  alizarine,  alizarine  green,  benzo  orange,  and  acid 
violet: it  adsorbs  fast  green,  acid  green,  and  croceine  orange 
considerably,  and  has  practically  no  adsorbing  action  on  emerald 
green  or  chrysoidine.  Red  phosphorus  will  decolorize  a  dilute 
solution  of  iodine  in  carbon  bisulphide.4 

Hatschek5  points  out  that  iodides  cannot  be  shown  to  be  present 
in  sea- water  by  any  of  the  usual  tests,  and  that  we  owe  our  supply 
of  iodine  to  the  adsorption  of  iodides  by  certain  sea-weeds. 

1  Cf.  OSTWALD:  Lehrbuch  allgem.  Chemie,  2nd.  Ed.,  1,  1093  (1891). 

2  Jour.  Chem.  Soc.,  28  170  (1875). 

3  Jour.  Phys.  Chem.,  17,  737  (1913). 

4  SESTINI:  Gazz.  chim.  ital.,  1,  323;  Jour.  Chem.  Soc.,  24,  1006  (1871). 
6  An  Introduction  to  the  Physics  and  Chemistry  of  Colloids,  6  (1913). 

90 


ADSORPTION  FROM  SOLUTION  91 

When  making  density  determinations1  by  weighing  a  solid  in  a 
solution,  an  error  may  be  introduced  because  of  the  solid  adsorb- 
ing some  of  the  salt.  This  error  is  likely  to  be  larger  the  finer 
the  particles  of  the  solid,  because  the  ratio  of  surface  to  mass 
increases  with  increasing  subdivision.  With  grains  of  quartz  or 
glass  varying  in  diameter  from  0.015  mm  to  0.9  mm,  placed 
in  solutions  of  iodides,  the  specific  gravity  of  the  latter  can  be 
adjusted  so  that  the  smaller  particles  will  sink  while  the  larger 
will  float.  Marble  is  so  porous  that  its  surface  is  almost  propor- 
tional to  its  volume,  and  consequently  grains  of  marble  do  not 
show  this  phenomenon. 

According  to  Skey2  antimony  and  arsenic  can  be  removed  so 
completely  from  solutions  of  their  oxides  or  chlorides  in  moder- 
ately strong  hydrochloric  acid  (with  a  little  tartaric  acid  in  the 
case  of  antimony)  by  charcoal  that  neither  of  them  can  be  de- 
tected therein  by  Reinsch's  test,  although  before  the  treatment 
they  could  be  detected  without  difficulty.  Commercial  sul- 
phuric and  hydrochloric  acids  diluted  with  a  little  water  can  be 
purified  from  either  of  these  substances  by  agitating  them  inter- 
mittently for  a  short  time  with  fresh  charcoal,  and  then  filtering 
off;  application  of  heat  to  the  mixture  expedites  this  result. 

Quite  recently  Lockemann  and  Paucke3  have  studied  quanti- 
tatively the  adsorption  of  arsenic  acid  by  charcoal,  hydrous 
aluminum  oxide,  hydrous  ferric  oxide,  and  albumin.  They  find 
that  egg  albumin  and  blood  serum,  when  coagulating,  carry  down 
certain  amounts  of  arsenic  from  aqueous  solutions.  Animal  char- 
coal also  adsorbs  some  amounts  of  arsenic  from  aqueous  solutions. 
When  hydrous  aluminum  oxide  is  precipitated  by  ammonia,  it 
carries  down  very  appreciable  quantities  of  arsenic.  Curiously 
enough  more  arsenic  is  carried  down  when  the  hydrous  aluminum 
oxide  is  precipitated  from  hot  solution  than  when  precipitated 
from  a  cold  one.  This  unexpected  fact  is  probably  due  to  the 
form  in  which  the  alumina  is  precipitated.  At  the  temperature  of 
the  water-bath,  arsenic  in  amounts  up  to  20  mg  As  per  100  cc 
can  be  precipitated  completely  but  this  requires  the  use  of  very 

1  THOULET:  Comptes  rendus,  99,  1072  (1884);  Jour.  Chem.  Soc.,  48,  476 
(1885). 

2  Chem.  News,  17,  157  (1868);  36,  6  (1877). 
8Zeit.  Kolloidchemie,  8,  273  (1911). 


92  APPLIED  COLLOID  CHEMISTRY 

Jarge  amounts  of  alumina.  When  precipitated  by  ammonia,  hy- 
drous iron  oxide  carries  down  large  amounts  of  arsenic,  more 
being  carried  down  the  lower  the  temperature  and  the  smaller 
the  excess  of  ammonia.  When  the  concentration  of  arsenic  does 
not  exceed  10  mg  As  per  100  cc  it  can  easily  be  precipitated 
quantitatively  by  means  of  iron. 

In  1845  Yorke1  discussed  the  adsorption  of  lead  oxide  by 
filter  paper,  while  Mil  Her2  and  Weiske3  have  shown  that  filter 
paper  takes  up  such  appreciable  quantities  of  barium,  strontium 
and  calcium  hydroxides  that  dilute  solutions  of  these  salts  should 
not  be  filtered  if  they  are  to  be  used  for  quantitative  work. 

A  special  phenomenon  in  connection  with  adsorption  by  filter- 
paper  has  been  studied  by  Bay  ley,4  Lloyd,5  Gordon,6  and  others.7 
If  a  drop  of  a  salt  solution  be  allowed  to  fall  on  filter-paper  the 
solution  will  tend  to  spread.  If  the  salt  is  adsorbed  very  strongly 
it  will  not  spread  and  we  get  a  relatively  wide  water-ring.  If 
the  salt  is  not  much  adsorbed,  it  will  spread  as  far  as  the  water. 
If  a  strip  of  filter-paper  be  dipped  into  a  solution  we  get  a  similar 
phenomenon;  the  water  will  rise  much  farther  than  the  salt  if 
the  latter  is  adsorbed  strongly.  Gordon  found  that  with  a 
M/40  CuS(>4  solution  the  water  rose  about  9.2  cm  in  half  an  hour 
and  the  copper  salt  only  5.5  cm.  With  a  M/2  solution  the  copper 
salt  rose  9.2  cm  and  so  did  the  water.  Ordinary  ink  shows  the 
phenomenon  well  with  blotting-paper. 

Leighton8  has  determined  the  adsorption  of  caustic  soda,  hy- 
drochloric acid,  sulphuric  acid,  and  phosphoric  acid  by  purified 
absorbent  cotton.  These  experiments  were  interesting  in  two 
ways.  •  In  the  first  place,  they  showed  that  no  compounds  were 
formed.  In  the  second  place,  data  were  obtained  showing  the 
possible  error  when  determining  adsorption.  The  usual  method 
is  to  shake  a  known  solution  with  a  weighed  amount  of  the 
adsorbed  solid,  pipette  off  some  of  the  supernatant  liquid,  and 

1  Mem.  Chem.  Soc.,  2,  399  (1845). 

2  Jour,  prakt.  Chem.,  83,  384  (1861). 

3  Jour.  Chem.  Soc.,  30,  662  (1876). 
*  Ibid.,  33,  304  (1878). 

6  Chem.  News,  61,  51  (1885). 

6  Jour.  Phys.  Chem.,  18,  337  (1914). 

7  See  SAHLBLOM:  Kolloidchemische  Beihefte,  2,  79  (1910). 

8  Jour.  Phys.  Chem.,  20,  188  (1916). 


ADSORPTION  FROM  SOLUTION 


93 


analyze.  The  amount  of  adsorption  is  calculated  from  the  de- 
crease in  concentration.  This  calculation  is  accurate  only  in 
case  no  liquid  is  taken  up  by  the  solid  adsorbing  agent.  If  the 
cotton  adsorbed  the  alkali  and  the  water  in  the  same  ratio  in 
which  they  occurred  in  the  solution  there  would  be  no  change  in 
the  concentration  of  the  solution  and  the  apparent  adsorption 
would  be  zero.  To  avoid  this  error,  Leighton  centrifuged  the  cot- 
ton for  an  hour  in  a  centrifugal  rotating  about  4000  revolutions  per 
minute,  and  then  analyzed  the  cotton.  This  direct  or  "gravi- 


0  0.20         o.40         o.eo         ago 

GRAMS  No  OH  ADSORBED  /=£>?  G&AM  COTTON 

FIG.  5. 

metric"  method  will  give  too  high  adsorption  unless  the  centri- 
fuging  removes  all  the  unadsorbed  solution.  On  the  other  hand, 
the  indirect  or  "titration"  method  gives  too  low  results  unless  no 
water  is  adsorbed  by  the  cotton  at  all.  Since  this  last  is  not 
true,  the  results  by  the  titration  method  are  necessarily  wrong 
with  cotton,  while  the  results  by  the  gravimetric  method  may 
be  right.  The  differences  between  the  two  methods  may  be 
quite  considerable.  With  a  solution  of  200  grams  NaOH  per 
liter  Leighton  found  an  adsorption  of  0.27  grams  NaOH  per 
gram  cotton  by  the  titration  method,  and  of  0.75  gram  NaOH  by 


94 


APPLIED  COLLOID  CHEMISTRY 


the  gravimetric  method.  The  true  adsorption  lies  between  these 
two  values.1  In  Tables  XXI-XXIV  are  given  Leighton's  data 
which  are  also  reproduced  graphically  in  Figs.  5-7. 

700  i 1 1 ff 1 1  700 


600 


5       10       15      20      25 

AWS  HZ  SO*    ADSORBED 
RER  JQQ  GRAMS    COTTON 

FIG.  6. 


$00 


10        15       20 

GRAMS  HyF*O+    ADSORBED 
JOO  GRAMS    COTTOW 


FIG.  7. 


Kellner2  has  studied  the  adsorption  of  acids  and  alkalies  by 
platinum  black.  One  gram  of  platinum  black  adsorbed  about 
20  percent  of  the  hydrochloric  acid  in  50  cc  of  a  thousandth- 

iSee  WEISER  and  SHERRICK:  Jour.  Phys.  Chem.,  23,  205  (1919). 
2  Wied.  Ann.,  67,  79  (1895). 


ADSORPTION  FROM  SOLUTION  95 

TABLE  XXI. — ADSORPTION  OF  SODIUM  HYDROXIDE  BY  COTTON 

Volume  of  solution  =  100  cc. 
Weight  of  cotton  =  about  1.0  g. 
Time  of  run  =  3  hours. 


Solution 
grams 
NaOH  per 
liter 

Grams  NaOH  adsorbed  per 
gram  cotton 

Solution 
grams  NaOH 
adsorbed 

Grams  NaOH  adsorbed  per 
gram  cotton 

Gravimetric 

Titration 

Gravimetric 

Titration 

474 

0.916 

230 

0.664 

450 

0.28 

212 

0.662 

427 

0.887 

210 

0.19 

415 

0.875 

190 

0.594 

400 

0.27 

189 

0.579 

379 

0.846 

184 

0.572 

350 

0.27 

175 

0.12 

323 

0.782 

140 

0.08 

300 

0.27 

138 

0.461 

276 

0.733 

125 

0.433 

275 

0.25 

90 

0.01 

231 

0.665 

40 



0.00 

TABLE  XXII. — ADSORPTION  OF  SULPHURIC  ACID  BY  COTTON 

Volume  of  solution  =  100  cc. 

Weight  of  cotton  about  1.0  g. 

Time  of  run  =  3  hours.  • 


Adsorption 


Solution  g  H2SO4  per  liter 

Gravimetric:  g  HzSO4  per 
100  g  cotton 

Titration:  g  HzSO4  per 
100  g  cotton 

747  7 

2.04 

521.7 
459  2 

23.70 

1.53 

0  12 

413.9 

20.99 

0.0 

352.3 

15.36 

0.0 

302.3 

12.55 

0.0 

252.3 

10.00 

0.0 

202.3 

7.35 

0.0 

151.7 

5.77 

0.0 

101.2 
50  0 

4.65 

0.0 
0.0 

96 


APPLIED  COLLOID  CHEMISTRY 


TABLE  XXIII. — ADSORPTION  OP  PHOSPHORIC  ACID  BY  COTTON 

Volume  of  solution  =  100  cc. 
Weight  of  cotton  about  1.0  g. 
Time  of  run  =  3  hours. 


Adsorption 

Solution  g  HsPCh  per  liter 

Gravimetric:  g  HsPO4  per 
100  g  cotton 

Titration:  g  HsPOi  per 
100  g  cotton 

715.2 

18.68 

0 

514.4 

18.43 

0 

382.9 

11.53 

0 

318.8 

8.57 

0 

158.0 

4.53 

0 

TABLE  XXIV. — ADSORPTION  OF  HYDROCHLORIC  ACID  BY  COTTON 

Volume  of  solution  =  100  cc. 
Weight  of  cotton  about  1.0  g. 
Time  of  run  =  3  hours. 


Adsorption 


Solution  g  HC1  per  liter 

Gravimetric:  g  HC1  per 
100  g  cotton 

Titration:  g  HC1  per 
100  g  cotton 

414.0 

13.  971 

373.6 

9.1 

11.  631 

331.3 

7.3 

9.281 

288.9 

6.9 

7.001 

209.2 

6.2 

2.34 

114.5 

3.1 

0 

45.8 

2.0 

normal  solution  (about  0.36  mg  HC1).  The  adsorption  of  caus- 
tic potash  is  greater.  The  bivalent  bases  and  acids  are  adsorbed 
in  general  more  than  the  univalent  ones.  Osaka2  has  made 
some  experiments  on  the  adsorption  of  potassium  and  sodium 
salts  by  blood  charcoal.  The  charcoal  takes  up  about  85  percent 
of  its  weight  of  water  when  suspended  in  saturated  water  vapor 
at  25°.  If  no  account  is  taken  of  this  fact,  a  very  large  error 

1  Probably  wrong  owing  to  volatilization  of  HC1. 

2  Mem.  Coll.  Sci.  Kyoto  Univ.,  1,  257  (1915). 


ADSORPTION  FROM  SOLUTION 


97 


may  be  introduced.  When  allowance  was  made  for  the  amount 
of  water  adsorbed,  it  was  found  that  the  adsorption  with  salts  of  the 
same  base  decreased  in  the  order  I  >  NO3  >  Br  >  Cl  >  SO4.  The 
results  are  shown  graphically  in  Fig.  8,  the  abscissas  being  milligram 
equivalents  of  salt  adsorbed  per  gram  charcoal  and  the  ordinates 
the  concentration  of  the  solution  in  gram  equivalents  per  liter. 
It  will  be  noticed  that  potassium  salts  are  adsorbed  more  strongly 
than  the  corresponding  sodium  salts.  The  numerical  values  are 
given  in  Table  XXV.  The  percentage  adsorptions  for  normal 
solutions  of  KI,  KNO3,  KBr,  KC1,  and  K2S04  are  approximately 
28,  16,  14,  7,  and  5  respectively.  Rona  and  Michaelis1  find  that 


1.1  1.5  1.5 

Of  SALT    AOSOf?B£D 


1.7 


Fio.  8. 

the  order  of  adsorption  of  the  anions  is  OH>CNS>1>NO3> 
Br  >  Cl  >  HPO4  >  SO4  and  of  the  cations  is  H  >  Al  >  Cu  >  Zn  >  Mg, 
Ca>NH4,  K,  Na. 

With  powdered  quartz  having  an  estimated  surface  of  1000cm2 
per  gram,  L.  J.  Briggs2  found  that  100  grams  quartz  adsorbed 
approximately  the  following  amounts  for  M/10  solutions:  Na- 
OH,  KOH  and  NH4OH,  0.8  millimols;  Na2CO3,  K2CO3,  (NH4)j- 
C03,  0.2  millimols;  NH4C1,  MgCl2,  NaCl,  NaNO3,  Na2S04, 
nothing. 

An  interesting  and  important  case  of  selective  adsorption  has 
been  worked  out  by  Lloyd.  From  fuller's  earth  he  has  prepared 
a  hydrous  aluminum  silicate3  which  adsorbs  alkaloids.  It  has 
been  suggested  that  the  material  be  called  Lloyd's  reagent. 

1  Biochem.  Zeit.,  94,  240  (1919). 

2  Jour.  Phys.  Chem.,  9,  623  (1905). 

3  WALDBOTT:  Jour.  Am.  Chem.  Soc.,  35,  837  (1913). 

7 


98  APPLIED  COLLOID  CHEMISTRY 

TABLE  XXV. — ADSORPTION  OF  SALTS  BY  BLOOD  CHARCOAL 
Milligrams  and  milligram  equivalents  per  gram  charcoal 


Cone,  of 
solution  in 
g  equiv./ 
liter 

Salt  adsorbed  by 
one  gram  charcoal 

Cone,  of 
solution  in 
g  equiv./ 
liter 

Salt  adsorbed  by 
one  gram  charcoal 

mg 

mg  equiv. 

mg 

mg  equiv. 

Potassium  sulphate 

Sodium  sulphate 

1.010 

51.4 

0.59 

1.068 

40.5 

0.57 

0.830 

45.1 

0.52 

0.933 

37.4 

0.53 

0.673 

37.0 

0.42 

0.753 

31.9 

0.45 

0.509 

29.3 

0.34 

0.583 

25.5 

0.36 

Potassium  chloride 

Sodium  chloride 

0.966 

68.6 

0.92 

0.998 

47.4 

0.81 

0.765 

50.8 

0.68 

0.896 

44.9 

0.77 

0  562 

41.8 

0.56 

0.697 

35.3 

0.60 

0.470 

33.2 

0.45 

0.597 

29.0 

0.49 

Potassium  nitrate 

Sodium  nitrate 

0.977 

151.2 

1.50 

0.978 

116.3 

1.37 

0.817 

129.9 

1.28 

0.816 

107.6 

1.27 

0.654 

112.7 

1.11 

0.647 

99.4 

1.03 

0.491 

87.7 

0.87 

0.484 

69.6 

0.82 

Potassium  iodide 

Potassium  bromide 

0.967 

277.9 

1.67 

0.970 

139.2 

1.17 

0.833 

250.8 

1.51 

0.717 

98.1 

0.82 

0.622 

207.4 

1.25 

0.526 

75.7 

0.64 

0.464 

167.8 

1.01 

Carey  Lea1  has  shown  that  silver  iodide  adsorbs  iodine  strongly  • 
This  property  of  silver  iodide  was  verified  by  shaking  portions  of 
freshly  precipitated  and  still  moist  Agl  with  iodine  solutions. 
An  alcoholic  solution  of  iodine,  diluted  until  it  has  a  pale  sherry 
wine  color  is  quickly  decolorized  by  Agl,  and  the  same  thing 
happens  with  a  very  dilute  solution  of  iodine  in  KE,  which  in  a 
few  minutes  becomes  as  colorless  as  water.  It  is  probably  this 
adsorption  which  makes  silver  iodide  photographic  emulsions 
apparently  less  sensitive  than  the  silver  bromide  emulsions.  If 
so,  it  should  not  be  difficult  to  overcome  this  trouble. 

Reinders2  has  found  that  when  silver  chloride   crystallizes 

1  Am.  Jour.  Sci.  (3),  33,  492  (1887). 
2Zeit.  phys.  Chem.,  77,  696  (1911). 


ADSORPTION  FROM  SOLUTION  99 

from  an  ammoniacal  solution  to  which  gelatine  has  been  added, 
the  crystals  contain  adsorbed  gelatine.  This  fact  is  of  great 
importance  for  the  theory  of  the  photographic  emulsion.  Much 
other  work  on  adsorption  by  crystals  is  to  be  found  in  the  papers 
of  Marc.1  The  selective  nature  of  the  adsorption  is  shown  by  the 
fact  that  barium  sulphate  carries  down  much  larger  amounts  of 
barium  nitrate  than  of  barium  chloride.2 

In  his  presidential  address  before  the  mathematical  and  phys- 
ical section  of  the  British  Association,  Trouton3  says:  "It  is 
a  very  well  known  experiment  in  adsorption  to  run  a  solution 
such  as  that  of  permanganate  of  potash  through  a  filter  of  sand, 
or,  better,  one  of  precipitated  silica,  so  as  to  provide  a  very 
large  surface.  The  first  of  the  solution  to  come  through  the 
filter  has  practically  lost  all  its  salt  owing  to  have  been  adsorbed 
by  the  surface  of  the  sand. 

"I  was  interested  in  finding  a  few  months  ago  that  Defoe, 
the  author  of  Robinson  Crusoe,  in  one  of  his  other  books,  depicts 
a  party  of  African  travellers  as  being  saved  from  thirst  in  a 
place  where  the  water  was  charged  with  alkali  by  filtering  the 
water  through  bags  of  sand.  Whether  this  is  a  practical  thing 
or  not  is  doubtful,  or  even  if  it  has  ever  been  tried;  for  it  is 
only  the  first  part  of  the  liquid  to  come  through  the  filter  which 
is  purified,  and  very  soon  the  surface  has  taken  up  all  the  salt 
it  can  adsorb  and  after  that,  of  course,  the  solution  comes 
through  intact.  It  is  interesting,  however,  to  know  that  so  long 
ago  as  Defoe's  time  the  phenomenon  of  adsorption  from  salt 
solutions  had  been  observed.  It  is  not  so  well  known,  that  in 
the  case  of  some  salts  under  the  circumstances  mentioned  above, 
the  first  of  the  solution  to  come  through  the  sand  filter  is  stronger 
instead  of  weaker.  This,  as  already  mentioned,  is  because  water, 
or  at  least  a  weaker  solution  forms  the  adsorption  layer." 

On  or  just  back  of  the  beaches  at  Galveston,  Nantucket,  and 
elsewhere,  there  are  wells  of  fresh  water,  the  levels  of  which 
correspond  approximately  to  the  ocean  level  at  high  tide.  It 
is  often  believed  that  the  water  in  these  wells  comes  from  the 
sea  and  that  the  salts  have  been  removed  in  some  mysterious 

1  Zeit.  phys.  Chem.,  61,  385  (1908);  67,  470;  68,  104  (1909);  73,  685  (1910)- 

2  WEISER  and  SHERRICK:  Jour.  Phys.  Chem.,  23,  205  (1919). 

3  Brit.  Ass.  Reports,  84,  287  (1914). 


100  APPLIED  COLLOID  CHEMISTRY 

way.1  Unfortunately  this  is  not  true.  The  water  comes  from 
inland  but  is  backed  up  by  the  ocean  and  does  not  flow  off  so 
rapidly  as  it  otherwise  would.  If  for  any  reason  the  outward 
flow  is  checked  for  a  sufficient  time,  the  salt  from  the  ocean  does 
work  back  into  the  wells.  This  has  been  noticed  at  Galveston 
when  excessive  amounts  of  water  are  drawn  from  the  wells. 


THE  ADSORPTION  ISOTHERM 

In  all  cases  which  have  been  studied  quantitatively  the  form 
of  the  adsorption  isotherm  is  similar  to  that  obtained  for  the 
adsorption  of  gases  by  solids.  Consequently  an  equation  of  the 
same  type  will  represent  both  sets  of  phenomena.  For  solutions 
we  write  the  equation 

(x/m)n  =  kc 

where  x  is  the  amount  adsorbed  by  m  units  of  the  solid  adsorbing 
agent,  c  is  the  concentration  of  the  solution,  and  n  is  not  neces- 
sarily an  integer  though  experimentally  never  less  than  unity. 
The  approximate  accuracy  of  the  formula  is  shown  in  Freund- 
lich's  data,2  Tables  XXVI— XXVIII.  Other  adsorption  formu- 
las have  been  proposed  or  discussed  by  Freundlich,3  McBain,4 
Schmidt,5  von  Georgievics,6  and  others;7  but  no  formula  is 
strictly  accurate  and  the  simplest  one  seems  the  best  for  the  time 
being.  If  the  curve  really  runs  parallel  eventually  to  the  axis 
of  ordinates,  the  simple  formula  is  necessarily  wrong. 

That  this  really  does  happen  seems  to  be  shown  very  conclu- 
sively in  some  experiments  by  Schmidt8  on  the  adsorption  of 
acetic  acid  by  charcoal,  Table  XXIX.  The  data  are  plotted  in 

1  HILGARD:  Soils,  267  (1906). 

2  Kapillarchemie,  147  (1909);  See  also  VON  GEORGIEVICS:  Monatshef t f ur 
Chemie,  34,  733  (1913). 

3  Zeit.  phys.  Chem.,  67,  385;  69,  284  (1907). 
<  Jour.  Chem.  Soc.,  91,  1683  (1910). 

5  Zeit.  phys.  Chem.,  74,  689  (1910);  77,  641  (1911);  78,  667  (1912);  91, 
103  (1916). 

6  Ibid.,  83,  269  (1914). 

7  MARC:  Zeit.   phys.    Chem.,    81,    641    (1913);    DIETL:  Monatsheft   fur 
Chemie,  36,  784  (1914);  TRUMPLER:  Zeit.  Kolloidchemie,  15,  10  (1914). 

8  Zeit.  phys.  Chem.,  77,  650  (1911). 


ADSORPTION  FROM 


101 


TABLE   XXVI. — DISTRIBUTION    OF    ACETIC 

BLOOD  CHARCOAL 
(x/m)n  =  kc. 
n  =  2.35;  log  k  =  0.98. 
c  in  mols  per  liter;  x/m  in  millimols  per  gram  charcoal. 


P  IjijTVEftN,  »JVfr'ATER''*AND 
•••••     '•'  •  *•  -  J>  ••  "• 


c 

x/m  found 

x/m  calc. 

logk 

0.0181 

0.467 

0.474 

0.966 

0.0309 

0.624 

0.623 

1.029 

0.0616 

0.801 

0.798 

0.984 

0.1259 

1.11 

1.08 

1.006 

0.2677 

1.55 

1.49 

1.030 

0.4711 

2.04 

1.90 

1.055 

0.8817 

2.48 

2.48 

0.981 

2.785 

3.76 

4.04 

0.906 

TABLE  XXVII. — DISTRIBUTION  OF  BROMINE  BETWEEN  WATER  AND  BLOOD 

CHARCOAL 
(x/m)n  =  kc. 
n  =  2.4;  logk  =  0.974. 
c  in  millimols  per  liter, 
x/m  in  millimols  per  gram  charcoal. 


c 

x/m  found 

x/m  calc. 

logk 

0.92 

2.07 

2.09 

0.965 

2.59 

3.10 

2.96 

1.032 

6.69 

4.27 

4.10 

1.028 

17.08 

5.44 

5.64 

0.930 

29.75 

6.80 

6.80 

0.974 

TABLE  XXVIII. — DISTRIBUTION  OF  BENZOIC  ACID  BETWEEN  WATER  AND 

BLOOD  CHARCOAL 
(x/m)n  =  kc. 
n  =  2.53;  logk  =  2.32. 
c  in  millimols  per  liter, 
x/m  in  millimols  per  gram  blood  charcoal. 


c 

x/m  found 

x/m  calc. 

logk 

6.18 

0.437 

0.445 

2.299 

25.00 

0.780 

0.776 

2.329 

53.13 

1.04 

1.04 

2.318 

117.73 

1.44 

1.43 

2.330 

102 


APPI}I&V  COLLOID  CHEMISTRY 

ADSORPTION  OP  ACETIC  ACID  BY  CHARCOAL 


100  cc  acetic  acid  solution  and  10  grams  charcoal. 
Ci  =  grams  acetic  acid  per  100  cc. 
c2  =  grams  acetic  acid  adsorbed. 


Sugar  charcoal 

Animal  charcoal 

Cl 

C2 

Cl 

C2 

0.009 

0.052 

0.04 

0.20 

0.032 

0.101 

0.19 

0.49 

0.037 

0.126 

1.20 

1.19 

0.212 

0.322 

1.88 

1.42 

1.16 

0.588 

3.18 

1.68 

3.76 

0.795 

5.09 

1.96 

3.75 

0.811 

13.63 

2.37 

5.60 

0.828 

14.39 

2.47 

9.18 

0.901 

21.52 

2.56 

12.65 

0.953 

32.06 

2.44 

16.60 

0.904 

38.89 

2.50 

25.73 

0.882 

29.38 

0.902 

30.60 

0.903 

Fig.  9.  In  both  these  cases  people  who  only  looked  at  the  last 
five  or  six  figures  might  easily  decide  that  a  definite,  compound 
was  formed  though  it  would  have  to  be  a  different  one  in  each 
case.  If  one  assumes  that  the  animal  charcoal  is  only  20  per- 
cent carbon,  the  hypothetical  compound  would  be  C2o(CH3CO2H). 
By  assigning  a  higher  molecular  weight  to  carbon,  the  formula  can 
be  made  anything  one  pleases. 

Schmidt1  does  not  always  give  the  units  which  he  uses  and  his 
manipulation  of  decimal  points  is  erratic;  but  he  finds  that  the 
equation 

(x/m)4  =  kc 

represents  pretty  well  the  distribution  of  iodine  between  charcoal 
and  alcohol  or  benzene  over  the  range  covered  by  the  experiments. 
For  -charcoal,  water,  and  acetic  acid  or  succinic  acid,  the  equation 
takes  the  form 

(x/m)2  =  kc 
1  Zeit.  phys.  Chem.,  15,  56  (1894). 


ADSORPTION  FROM  SOLUTION 


103 


while  the  exponent  becomes  10  in  the  case  of  oxalic  acid.  For 
the  distribution  of  potassium  chloride  between  silicic  acid  and 
water  the  exponent  is  unity,  the  equation  becoming 

(x/m)  =  kc 

The  same  equation  describes  the  distribution  of  potassium  chlo- 
ride between  stannic  oxide  and  water1  and  between  charcoal 

40 


16 


• 

k 

< 

SI 

SfSGArt, 

> 

AM/&7AL 
CHARCOAL 

CHAffCOAL 

y 

0 

P 

<o 

o 

^  £ 

"l 

fcs 

f 

<^ 

A 

^  X 

(W 

/ 

3               C 

.4                  0 

^  —  — 

3 

—      -*• 
I. 

o  
2             1. 

^ 

3             Z. 

0             P. 

4 

G  ft  A  MS  AC£77C  ACJ&     ADSOf?G£D    f^£F^ 

JO    Gf?AM3      CHARCOAL 

FIG.  9. 

and  water.2  A  glance  at  Fig.  8  shows  that  Osaka's  data  for 
potassium  sulphate  and  potassium  nitrate  fall  very  nearly  on  a 
straight  line  and  could  therefore  be  given  approximately  by  this 
equation  over  the  range  plotted.  This  has  no  special  significance 
because  the  other  data  in  Fig.  8  distinctly  do  not  give  straight 
lines. 

Van  Slyke  and  Van  Slyke3  state  that  the  distribution  of  acids 
between  caseine  and  water  can  be  represented  by  the  adsorption 
isotherm,  the  exponent  being  1.95  for  sulphuric  acid. 


BEMMELEN:  Zeit.  anorg.  Chem.,  23,  113  (1900). 
*LACHS  and  MICHAELIS:  Zeit.  Elektrochemie,  17,  2  (1911). 
3  Am.  Chem.  Jour.,  38,  383  (1897). 


104  APPLIED  COLLOID  CHEMISTRY 

Georgievics1  finds  that  the  distribution  of  indigo  carmine 
between  silk  and  water  and  of  methylene  blue  between  cellulose 
and  water  can  be  represented  by  the  formula  (x/m) 2  =  kc,  while 
Ktister2  shows  that  the  distribution  of  iodine  between  starch 
and  water  can  be  described  by  the  formula  (x/m) 5  =  kc.  Kiis- 
ter's  experiments  prove  that  "starch  iodide"  so-called  is  not  a 
definite  compound.  They  also  furnish  a  serious  objection  to  the 
view  that  the  iodine  forms  a  solid  solution  in  starch  following 
the  same  laws  governing  the  distribution  of  a  substance  between 
two  liquid  layers.  If  that  were,  the  case  the  molecular  weight 
of  iodine  in  starch  would  have  to  be  one-fifth  the  molecular  weight 
of  iodine  in  water.  This  means  a  molecular  weight  of  about  51 
for  iodine  dissolved  in  starch,  which  is  improbable.  This  diffi- 
culty is  avoided  when  we  consider  the  phenomenon  as  one  of 
adsorption  because  there  is  no  known  relation  between  the  expo- 
nent in  the  formula  for  the  adsorption  isotherm  and  the  molecular 
weight  of  anything. 

Biltz3  has  shown  that  the  blue  color  obtained  when  iodine  is 
taken  up  by  basic  lanthanum  acetate  is  due  to  an  adsorption  of 
iodine  by  the  salt.  The  exponential  factor  is  2.  It  is  3  for  the 
distribution  of  alizarine  between  chromic  oxide  and  water.4 

Davis5  finds  that  the  exponential  formula  reproduces  satis- 
factorily enough,  the  data  for  the  distribution  of  iodine  between 
charcoal  and  various  organic  liquids.  He  considers  further 
that  the  adsorption  of  iodine  by  various  charcoals  consists  of  a 
surface  condensation  and  a  diffusion  (solid  solution)  into  the 
interior  of  the  carbon,  the  surface  condensation  being  nearly 
instantaneous  and  quite  complete  in  some  hours,,  while  the  diffu- 
sion proceeds  for  weeks  or  months.  This  conclusion  is  disputed 
by  Schmidt- Walter,6  who  believes  that  the  charcoal  acted  as  a 
catalytic  agent  causing  the  iodine  to  react  with  the  organic 
liquids. 

The  adsorption  of  milk  sugar  and  cane  sugar  by  charcoal  has 

1  Sitzungsber.  Akad.   Wiss.   Wien.,   103,  lib,   589  (1894);  104,  lib,  309 
(1895);  Zeit.  Farbenindustrie,  2,  253  (1903). 

2  Liebig's  Ann.,  283,  364  (1891). 

3  Ber.  deutsch.  chem.  Ges.,  37,  719  (1904). 

4  BILTZ:  Ber.  deutsch.  chem.  Ges.,  38,  4143  (1905). 
6  Jour.  Chem.  Soc.,  91,  1666  (1907). 

6  Zeit,  Kolloidchemie,  14,  242  (1914). 


ADSORPTION  FROM  SOLUTION  105 

been  studied  quantitatively  by  Wiegner,1  the  value  of  the  expo- 
nent ft  being  about  4.5  for  milk  sugar  and  a  little  less  than  8.2 
for  cane  sugar. 

Walker  and  Appleyard2  have  studied  the  very  interesting 
case  of  picric  acid  and  silk.  The  distribution  of  picric  acid 
between  silk  and  water  is  reversible  and  can  be  represented  by 
the  equation  (x/m)2-7  =  kc.  When  picric  acid  is  dissolved  in 
dry  benzene,  however,  dry  silk  does  not  adsorb  any  picric  acid 
from  the  solution,  nor  does  dry  silk,  dyed  with  picric  acid,  lose 
any  picric  acid  to  dry  benzene.  It  is  apparently  not  a  case  of 
reversible  equilibrium  and  Walker  and  Appleyard  were  unable  to 
account  for  it.  Quite  recently  a  tentative  explanation  has  been 
put  forward3  that  picric  acid,  like  many  other  dyes,  exists  in  two 
forms:  in  this  case  the  one  colored,  the  other  colorless.  In 
water  or  alcoholic  solution  the  two  forms  are  in  equilibrium,  and 
so  reversible  equilibrium  between  the  liquid  and  silk  exists.  But 
a  carbon  tetrachloride  solution  of  picric  acid,  at  least  at  ordinary 
temperatures  and  at  60°,  contains  so  little  of  the  colored  modifica- 
tion that  reversible  equilibrium  is  reached  with  difficulty.  The 
case  of  the  benzene  solution  is  complicated  by  presence  of  the 
benzene  picric  acid  compound,  but  this  probably  has  no  dyeing 
properties.  One  could  also  prophesy  the  same  results  with  ligroin 
or  chloroform  solutions — in  fact  with  any  colorless  solution  of 
picric  acid.  The  effect  of  solvent  on  tautomeric  change  is  exem- 
plified in  the  cases  of  nitroso-benzene  and  the  pseudonitrols, 
which  as  solids  are  colorless,  but  yield  colored  solutions. 

Cameron  and  Patten4  find  that  the  distribution  of  gentian 
violet  between  soils  and  water  or  of  eosine  between  quartz  and 
water  can  be  represented  fairly  well  by  exponential  formulas; 
but  that  flocculation  of  the  adsorbing  medium  is  apparently  a 
disturbing  factor.  That  flocculation  actually  does  take  place 
had  been  shown  by  Patten5  in  a  special  set  of  experiments  on 
quartz  flour  and  dyes. 

1  Zeit.  Kolloidchemie,  8,  126  (1911). 

2  Jour.  Chem.  Soc.,  69,  1334  (1896). 

3  BLUCHER  and  FARNAU:  Jour.  Phys.  Chem.,  18,  640  (1914).     Cf.  SISLEY: 
Bull.  Soc.  chim.,  Paris,  865  (1900). 

4  Jour.  Phys.  Chem.,  11,  581  (1907). 

6  Trans.  Am.  Electrochem.  Soc.,  10,  67  (1906). 


106 


APPLIED  COLLOID  CHEMISTRY 


In  Table  XXX  are  given  a  list  of  a  few  of  the  systems  which 
have  been  studied  quantitatively.  The  values  for  n  have  un- 
doubtedly been  rounded  off  in  most  cases. 

TABDE  XXX. — EXPONENTS  IN  ADSORPTION  ISOTHERMS 


Solvent 

Solute 

Solid 

n 

Observer 

Alcohol 

Iodine 

Charcoal 

4  0 

Schmidt1 

Benzene  
Water. 

Iodine 
Acetic  acid 

Charcoal 
Charcoal 

4.0 
2  0 

Schmidt1 
Schmidt1 

Water 

Succinic  acid 

Charcoal 

2  0 

Schmidt1 

Water  

Oxalic  acid 

Charcoal 

10.0 

Schmidt1 

Water. 

Potassium 

Silicic  acid 

1  0 

Schmidt1 

Water  

chloride 
Potassium 

Stannic  oxide 

1.0 

Van      Bemme- 

Water 

chloride 
Potassium 

Charcoal 

1  0 

len2 
Lachs  and  Mi- 

Water  

chloride 
Sulphuric  acid 

Caseine 

1  95 

chselis3 
Van  Slyke4 

Water 

Indigo  carmine 

Silk 

2  0 

Georgievics6 

Water  

Methylene 

Cellulose 

2.0 

Georgievics6 

Water 

blue 
Iodine 

Starch 

5  0 

Kuster6 

Water  

Alizarine 

Chromic  oxide 

3  0 

Biltz7 

Water  

Milk  sugar 

Charcoal 

4  5 

Wagner8 

Water 

Cane  sugar 

Charcoal 

8  2 

Wagner8 

Water  

Picric  acid 

Silk 

2.7 

Walker   and 

Appleyard9 

ABNORMAL  ADSORPTION 

A  still  more  striking  case  of  flocculation  and  consequent  change 
of  adsorption  was  studied  by  Lottermoser  and  Rothe10  in  the 

1  Zeit.  phys.  Chem.,  16,  56  (1894). 
2Zeit.  anorg.  Chem.,  23,  113  (1900). 

3  Zeit.  Elektrochemie,  17,  2  (1911). 

4  Am.  Chem.  Jour.,  38,  383  (1897). 

6  Sitzungsber.  Akad.   Wiss.  Wien.,   103,   116,  589  (1894);  104  116,  309 
(1895);  Zeit.  Farbenindustrie,  2,  253  (1903). 

6Liebig's  Ann.,  283,  264  (1891). 

7  Ber.  deutsch.  chem.  Ges.,  38,  4143  (1905). 

8  Zeit.  Kolloidchemie,  8,  126  (1911). 

9  Jour.  Chem.  Soc.,  69,  1334  (1896). 
"Zeit.  phys.  Chem.,  62,  359  (1908). 


ADSORPTION  FROM  SOLUTION  107 

adsorption  of  potassium  iodide  by  silver  iodide.  For  concen- 
trations of  potassium  iodide  in  water  up  to  about  M/300  the 
exponential  formula  holds  fairly  well,  the  exponential  factor 
being  a  little  over  13.  Beyond  this  concentration  the  amount  of 
adsorption  actually  decreases  with  increasing  concentration  in 
the  water  phase.  The  change  in  the  structure  of  the  silver  iodide 
can  easily  be  seen  under  the  microscope.  The  precipitate 
becomes  denser  and  more  granular  and  shows  signs  of  crystalliza- 
tion. On  the  other  hand  silver  nitrate  is  adsorbed  by  silver 
iodide  without  producing  this  change  within  the  concentrations 
studied.  The  exponential  factor  is  2  for  silver  nitrate.  The 
silver  iodide  shows  a  marked  tendency  to  change  in  structure 
and  great  care  is  necessary  if  one  wishes  to  prepare  two  samples 
with  the  same  adsorbing  power. 

Freundlich  and  Schucht1  found  that  amorphous  mercuric 
sulphide  changes  over  spontaneously  into  a  crystalline  form  and 
then  has  a  decreased  power  of  adsorbing  dyes.  The  adsorption 
of  eosine  by  hydrous  copper  oxide  varies  with  changes  in  the 
cupric  oxide.2  Wagner3  has  shown  that  when  the  salts  of  alumi- 
num, iron,  etc.,  are  hydrolyzed  they  adsorb  the  free  acid  to  some 
extent ;  but  that  in  time  the  adsorption  decreases  so  much  that 
practically  all  the  acid  is  set  free. 

Evans4  reported  a  number  of  cases  of  apparently  abnormal 
adsorption  by  filter  paper.  With  hydrochloric  acid  the  amount 
adsorbed  was  apparently  independent  of  the  concentration  of 
the  solution  from  0.007N  to  0.08N.  It  then  increased  to  double 
the  amount  as  the  concentration  of  the  solution  rose  to  0.28N 
and  dropped  off  apparently  to  zero  with  0.36N  HC1  with  normal 
acid.  In  a  second  series  the  maximum  adsorption  was  found  at 
about  0.2N  HC1  and  there  was  apparently  no  adsorption  with 
N/2  acid.  A  similar  behavior  was  observed  with  copper  sulphate 
solutions,  the  maximum  adsorption  occurring  at  about  0.04M 
CuSO4  and  the  adsorption  dropping  to  zero  for  a  0.13M  solu- 
tion. The  case  of  hydrochloric  acid  was  studied  by  Miss  Murray5 

1  Zeit.  phys.  Chem.,  86,  660  (1913). 

2  GILBERT:  Jour.  Phys.  Chem.,  18,  592  (1914). 

3  Monatsheft  fur  Chemie,  34,  95  (1913). 

*  Jour.  Phys.  Chem.,  10,  290  (1906). 

•  Ibid.,  20,  621  (1916). 


108  APPLIED  COLLOID  CHEMISTRY 

who  showed  that  normal  results  were  obtained  under  ordi- 
nary conditions  if  one  titrated  with  methyl  orange  as  indicator. 
If  one  used  phenolphthalein  as  indicator  and  ignored  the  presence 
of  carbonate  in  the  standard  alkali,  abnormal  results  were  ob- 
tained. Since  the  apparently  abnormal  adsorption  in  the  case 
of  hydrochloric  acid  was  proved  to  be  due  to  analytical  error,  it 
is  probable  that  the  measurements  with  copper  sulphate,  etc., 
are  not  absolutely  reliable. 

While  there  is  no  abnormal  adsorption  of  hydrochloric  acid 
under  ordinary  conditions,  Miss  Murray  found  that  the  adsorp- 
tion dropped  to  less  than  one-half  its  original  value  if  filter  paper 
were  allowed  to  stand  for  7-10  days  in  contact  with  hydrochloric 
acid  solutions.  The  nature  of  the  change  in  the  filter  paper  under 
these  conditions  is  not  known.  It  is  also  not  known  whether  a 
similar  change  is  to  be  observed  with  purified  absorbent  cotton. 

Biltz  and  Steiner1  found  abnormal  adsorption  of  night  blue 
and  Victoria  blue  by  cotton,  the  adsorption  passing  through  a 
maximum  with  increasing  concentration.  It  is  impossible  to 
say  to  what  extent  the  decreasing  adsorption  is  due  to  experi- 
mental error,  to  changes  in  the  adsorbing  power  of  the  cotton, 
and  to  other  substances  present  in  the  dye-bath. 

NEGATIVE  ADSORPTION 

Lagergren2  in  1899,  reported  the  negative  adsorption  of  some 
chlorides  and  of  ammonium  bromide  by  charcoal,  the  solution 
becoming  more  concentrated  after  being  shaken  with  the 
charcoal.  Quite  recently  Osaka3  found  that  sodium  nitrate, 
potassium  bromide,  potassium  iodide  and  potassium  nitrate 
are  adsorbed  positively  by  blood  charcoal  while  sodium  chloride, 
sodium  sulphate  and  potassium  sulphate  are  apparently  adsorbed 
negatively.  The  solution  of  potassium  chloride  is  adsorbed  posi- 
tively when  the  concentration  is  comparatively  high  and  is 
apparently  adsorbed  negatively  when  the  concentration  is  com- 
paratively low.  With  sodium  sulphate  the  increase  in  concen- 
tration was  about  1  percent  and  was  less  than  that  with  the 
other  salts. 

1  Zeit.  Kolloidchemie,  7,  113  (1910). 

2Freundlich:  Kapillarchemie,  165  (1909). 

3  Mem.  Coll.  Sci.  Kyoto  Univ.,  1,  257  (1915). 


ADSORPTION  FROM  SOLUTION  109 

Mathieu1  observed  negative  adsorption  with  a  number  of  dilute 
solutions  when  adsorbed  by  porous  plates,  membranes,  or  capil- 
lary tubes.  With  normal  solutions  the  concentrations  in  the 
capillary  tubes  were  often  only  one-tenth  that.  The  difference 
in  concentration  increases  with  decreasing  radius  of  the  capillary 
tubes  and  Mathieu  considers  it  quite  possible  that  with  very 
fine  tubes  water  alone  would  be  adsorbed,  a  conclusion  which, 
as  Mathieu  himself  points  out,  is  of  distinct  importance  for  the 
theory  of  semi-permeable  membranes.  If  this  conclusion  is 
true  and  general,  it  accounts  for  the  results  of  Bigelow2  and  of 
Bart  ell3  who  found  that  osmotic  phenomena  appeared  in  porous 
cups  when  the  pores  were  sufficiently  fine  or  were  clogged  suffi- 
ciently. It  is  clear  that  we  can  get  osmotic  phenomena  in  two 
distinct  ways,  depending  on  whether  we  have  a  continuous 
film  or  a  porous  one.  In  the  case  of  a  continuous  film,  it  is 
essential  that  the  solvent  shall  dissolve  in  the  membrane  and 
that  the  solute  shall  not.  Since  the  permeability  is  not  depend- 
ent on  adsorption,  there  is  no  reason  why  there  should  be  any 
fundamental  difference  between  the  adsorption  of  a  solute  which 
does  pass  through  the  membrane  and  of  one  which  does  not  pass 
through.  If  we  have  a  porous  film,  we  get  osmotic  phenomena 
only  in  case  the  pore  walls  adsorb  the  pure  solvent  and  the  diame- 
ter of  the  pores  is  so  small  that  the  adsorbed  film  of  pure  solvent 
fills  the  pores  full.  Under  these  circumstances  the  dissolved  sub- 
stance cannot  pass  through  the  membrane  unless  adsorbed  by  the 
latter.  There  is  therefore  an  important  difference  between  a 
solute  which  does  pass  through  a  porous  membrane  and  one  that 
does  not,  in  that  the  first  is  adsorbed  by  the  membrane  and  the 
second  is  not.  By  making  suitable  assumptions  this  distinction 
can  perhaps  be  eliminated.4 

Kahlenberg5  has  shown  that  benzene,  toluene,  and  pyridine 
pass  through  a  rubber  membrane  very  readily,  while  water 
does  not.  This  is  perfectly  natural  on  the  assumption  that  these 

1  Drude's  Ann.,  9,  340  (1902). 

2  Jour.  Am.  Chem.  Soc.,  29,  1576,  1675  (1907);  31,  1194  (1909). 

8  Jour.  Phys.  Chem.,  16,  659  (1911);  16,  318  (1912);  Jour.  Am.  Chem.  Soc., 
36,  646  (1914);  38,  1029,  1036  (1916). 
'See  Tinker:  Proc.  Roy.  Soc.  92A,  257  (1915). 
6  Jour.  Phys.  Chem.,  10,  141  (19C6). 


110  APPLIED  COLLOID  CHEMISTRY 

liquids  dissolve  in  rubber,  but  on  the  basis  of  pores  it  is  hard  to 
understand  why  water  should  not  pass  through.  One  might  cite 
the  case  of  the  oiled  sieve  and  say  that  water  does  not  wet  rubber, 
but  water  does  wet  rubber.  The  case  of  trichloracetic  acid  is 
also  interesting.  When  dissolved  in  benzene  it  passes  readily 
through  a  rubber  membrane,  whereas  it  passes  very  slowly  when 
dissolved  in  water.  This  is  as  it  should  be  if  we  are  dealing  with 
solution,  but  it  is  hard  to  explain  if  we  are  dealing  with  adsorption. 
While  nothing  is  proved,  I  am  inclined  to  think  that  a  rubber 
membrane  is  not  a  porous  one  in  the  sense  that  BartelFs  clogged 
plate  is,  and  1  believe,  therefore,  that,  when  a  rubber  film  acts  as  a 
semi-permeable  membrane,  the  solvent  dissolves  in  the  rubber 
and  passes  through  it  essentially  in  that  way.  There  are  no 
satisfactory  data  in  regard  to  the  copper  f errocyanide  membrane, 
but  it  seems  better  for  the  present  to  consider  it  as  like  the  rub- 
ber membrane. 

REVERSIBILITY  OF  EQUILIBRIUM 

To  be  certain  that  the  equilibrium  represented  by  the  adsorp- 
tion isotherm  is  a  reversible  one,  it  should  be  reached  from  both 
sides.  As  a  rule,  that  is  not  done,  but  Freundlich1  has  made  a 
couple  of  experiments  to  show  that  the  same  end-point  is  actu- 
ally reached.  For  the  distribution  of  acetic  acid  between  char- 
coal and  water,  he  obtained  a  concentration  in  the  water  of 
N/16.47  when  he  shook  one  gram  of  charcoal  with  100  cc  of 
N/ 14.53  solution,  and  a  concentration  of  N/ 16.49  when  he  shook 
one  gram  of  charcoal  with  50  cc  of  a  N/7.27  solution  and  then 
added  50  cc  water.  For  the  distribution  of  benzoic  acid  between 
charcoal  and  benzene,  a  similar  proceeding  with  other  concentra- 
tions gave  him  the  two  final  values  of  N/8.5  and  N/8.48.  In 
some  cases  equilibrium  is  reached  very  rapidly  from  both  sides, 
and  in  others  not.2  While  the  adsorption  equilibrium  is  theoret- 
ically a  reversible  one,  there  are  conditions  under  which  there 
may  be  apparent  or  actual  irreversibility.  If  charcoal  takes  a 
dye  out  of  solution  to  such  an  extent  as  to  decolorize  the  water 

1  Kapillarchemie,  148  (1909);  see  also  VON  SCHROEDER  :  Kolloidchemische 
Beihefte,  1,  14  (1909). 

2  FREUNDLICH:  Zeit.  phys.  Chem.,  67,  388  (1906);  OSTWALD:  Lehrbuch 
allgem.  Chemie,  1,  1096  (1891);  SCHMIDT:  Zeit.  phys.  Chem.,  74,  708  (1910). 


ADSORPTION  FROM  SOLUTION  111 

practically,  no  amount  of  washing  with  water  will  give  a  colored 
solution,  even  though  dye  is  being  taken  out  all  the  time.1  In 
this  case  the  adsorption  is  strictly  reversible,  and  the  difficulty 
was  in  the  interpretation  of  the  results. 

There  is  one  experiment  which  I  always  like  to  try,  because  it 
proves  something  whichever  way  it  goes.  A  solution  of  iodine 
in  water  is  shaken  with  bone-black,  filtered,  and  tested  with 
starch  paste.  If  the  colorless  solution  does  not  turn  the  starch 
blue,  the  experiment  shows  how  completely  charcoal  extracts 
iodine  from  aqueous  solution.  If  the  starch  turns  blue,  the 
experiment  shows  that  the  solution,  though  apparently  colorless, 
still  contains  iodine  which  can  be  detected  by  means  of  the  sensi- 
tive starch  test. 

If  the  adsorbing  substance  changes  through  agglomeration, 
crystallization,  or  in  any  other  way,  its  specific  adsorbing  power 
will  change,  and  the  adsorbed  substance  will  be  held  either  more 
or  less  firmly,  as  the  case  may  be.  Some  instances  of  this  have 
already  been  cited  under  abnormal  adsorption.  The  adsorbed 
substance  may  also  change  on  standing  or  on  heating,  in  which 
case  we  shall  have  an  apparent  irreversibility,  though  we  are 
really  dealing,  as  before,  with  a  new  reversible  equilibrium. 
This  case  occurs  very  frequently  in  dyeing.2 

SPECIFICITY  OF  ADSORPTION 

Since  adsorption  is  essentially  specific,  the  amount  of  adsorp- 
tion will  necessarily  vary  with  the  nature  of  the  adsorbing  agent, 
the  liquid,  and  the  substance  to  be  adsorbed.  In  illustration  of 
this,  Freundlich3  cites  the  experiments  of  Wohler,  Pluddeman 
and  Wohler4  to  the  effect  that  charcoal  and  ferric  oxide  adsorb 
benzoic  acid  about  ten  times  as  strongly  as  acetic  acid,  while 
chromium  oxide  adsorbs  the  two  acids  about  equally,  and  plati- 
num black  adsorbs  acetic  acid  a  little,  but  benzoic  acid  practically 
not  at  all.  All  salts  seem  to  show  distinct  adsorption  for  their 
own  ions.  Thus  silver  bromide  adsorbs  silver  nitrate  or  potas- 

1  FREUNDLICH  and  NEUMANN:  Zeit.  phys.  Chem.,  67,  538  (1900). 
2 LAKE:  Jour.  Phys.  Chem.,  20,  761  (1916). 

3  Kapillarchemie,  155  (1909). 

4  Zeit.  phys.  Chem.,  62,  664  (1908). 


112  APPLIED  COLLOID  CHEMISTRY 

sium  bromide,  but  not  potassium  nitrate.  Charcoal  adsorbs 
both  acid  and  basic  dyes.  Alumina  takes  up  many  acid  dyes 
readily  and  not  the  basic  dyes ;  silica  and  tannin  adsorb  the  basic 
dyes  more  readily  than  acid  dyes.  Wool  adsorbs  many  dyes 
strongly  without  a  mordant  and  cotton  relatively  few.  What  is 
known  as  Schulze' s  law1  is  that  the  power  of  active  ions  to  pre- 
cipitate colloidal  solutions  is  a  function  of  their  valence,  or  of 
the  number  of  electrical  charges  which  they  carry.  Since  the 
precipitating  power,  in  the  cases  studied  by  Schulze,  depends 
on  the  degree  of  adsorption,  it  follows  that,  in  so  far  as  Schulze' s 
law  holds,  a  trivalent  ion  will  be  adsorbed  more  strongly  than  a 
bivalent  ion,  and  the  latter  more  strongly  than  a  univalent  one. 
Schulze's  law  is  merely  a  first  approximation.  Everybody  recog- 
nizes that  hydrogen  and  hydroxyl  ions  are  not  to  be  classed  with 
the  other  univalent  ions,  because  they  are  usually  adsorbed  much 
more  strongly,2  and  everybody  recognizes  that  there  are  other 
exceptions.  In  case  of  doubt,  it  is  generally  safe  to  assume  that 
an  ion  of  higher  valence  will  be  adsorbed  more  strongly  than  one 
of  lower  valence,  but  it  is  a  mistake  to  consider  this  so-called 
law  as  anything  more  than  a  guide.  For  instance,  Osaka3  found 
that  charcoal  adsorbed  potassium  salts  in  the  following  order 
when  the  salts  were  present  in  equivalent  concentrations:  KI> 
KNO3>KBr>KCl>K2SO4,  from  which  it  follows,  that,  for 
equivalent  concentrations,  the  sulphate  ion  is  adsorbed  the  least 
of  all.  If  we  were  to  compare  molecular  concentrations  the 
sulphate  ion  would  be  adsorbed  as  strongly  as  the  bromide  ion, 
but  less  than  the  nitrate  or  the  iodide  ion.  Davis4  found  that 
the  order  of  adsorption  of  iodine  from  different  liquids  was  not 
the  same  with  different  kinds  of  charcoal.  With  animal  char- 
coal there  was  decreasing  adsorption  in  the  order:  chloroform, 
alcohol,  ethyl  acetate,  benzene,  and  toluene;  with  sugar  charcoal 
the  adsorption  decreased  in  the  order:  chloroform,  toluene, 
ethyl  acetate,  benzene,  and  alcohol;  whereas  for  cocoanut  char- 
coal the  order  was  toluene,  chloroform,  benzene,  alcohbl,  and  ethyl 
acetate.  There  are  at  least  two  factors  governing  the  effect  of 

1  SCHULZE:  Jour,  prakt.  Chem.  (2)  25,  43  (1882);  27,  320  (1884). 

2  FREUNDLICH  :  Kapillarchemie,  354  (1909).    . 

3  OSAKA:  Mem.  Coll.  Sci.,  Kyoto,  1,  267  (1915). 

4  Jour.  Chem.  Soc.,  91,  1682  (1907). 


ADSORPTION  FROM  SOLUTION  113 

the  solvent.  The  more  soluble  the  dissolved  substance  is  in  a 
given  solvent,  the  less  readily  will  it  be  adsorbed,  provided  we 
can  neglect  the  adsorption  of  the  solvent  itself  by  the  solid. 
There  are  many  illustrations  of  this,  but  one  will  suffice.  Char- 
coal will  decolorize  aqueous  solutions  of  iodine  or  of  methyl  violet, 
but  alcohol  will  extract  the  color  from  the  charcoal.  The  solu- 
bility cannot  be  the  sole  factor,  however,  because  then  the  sol- 
vents could  always  be  arranged  in  the  same  order  for  the  same 
solute,  regardless  of  the  nature  of  the  adsorbing  agent.  This  is 
disproved  absolutely  by  the  experiments  of  Davis.  One  other 
factor  is  the  adsorption  of  the  solvent  by  the  adsorbing  agent. 
This  factor  was  not  taken  into  account  at  all  by  Davis,  whose  data 
are  therefore  not  sufficient  to  enable  us  to  tell  whether  there  are 
other  factors  to  be  considered. 

In  so  far  as  adsorption  is  accompanied  by  an  evolution  of  heat, 
the  amount  of  adsorption  must  decrease  with  rising  temperature. 
This  is  found  to  be  the  case  experimentally,  but  the  change  is 
often  a  very  small  one.  In  some  cases  there  is  an  apparently 
large  increase  in  adsorption  with  rising  temperature.  Thus 
wool  takes  up  very  little  acid  violet  at  20°  and  a  great  deal  at 
about  95°.  Experiments  showed1  that  this  adsorption  is  not  re- 
versible, for  the  acid  violet  adsorbed  at  the  higher  temperature 
cannot  be  washed  out  to  any  extent  at  20°.  After  adsorption 
the  dye  agglomerates  or  changes  so  that  it  becomes  practically 
insoluble  and  consequently  the  wool  takes  up  more  dye. 

ADSORPTION  OF  SEVERAL  SOLUTES 

Some  years  ago  there  was  a  widespread  rumor  that  strychnine 
had  been  added  to  a  certain  breakfast  cereal  to  make  it  more 
appetizing.  The  report  was  false  and  not  even  new,  for  in  1852  it 
was  alleged  that  strychnine  was  being  added  to  certain  English 
pale  ales.  In  order  to  show  that  strychnine  could  be  detected 
in  beer  if  present,  Graham  and  Hofmann2  shook  two  ounces  of 
animal  charcoal  with  half  a  gallon  of  beer  to  which  1/2  grain 
of  strychnine  had  been  added.  The  strychnine  was  removed 
practically  completely  by  the  charcoal,  and  was  extracted  from 
that  with  alcohol  and  identified. 


Jour.  Phys.  Chem.,  20,  761  (1916). 
2  Jour.  Chem.  Soc.,  6,  713  (1853). 
8 


114  APPLIED  COLLOID  CHEMISTRY 

Skey1  reports  that  dilute  sulphuric  acid  can  be  freed  from  traces 
of  nitric  acid  by  shaking  with  charcoal,  while  concentrated  sul- 
phuric acid  cannot  be.  Dudley2  showed  that  the  rank  and  dis- 
agreeable odor  of  raw  whiskey  can  be  removed  by  leaching 
through  charcoal.  Schmidt3  has  studied  the  simultaneous 
adsorption  of  iodine  and  acetic  acid  by  charcoal  from  solution  in 
water  and  in  ethyl  acetate.  With  both  solvents  less  of  each 
substance  was  adsorbed  than  if  the  other  had  not  been  present. 
Schmidt  believes  that  this  is  general,  but  this  statement  is  un- 
doubtedly too  broad,  though  it  holds  in  many  cases.  Preund- 
lich  and  Masius4  studied  the  adsorption  of  pairs  of  organic  acids 
and  obtained  results  similar  to  those  of  Schmidt.  They  also 
found  that  the  acid,  which  is  adsorbed  more,  is  displaced  less 
when  the  two  acids  are  present  in  the  solution.  These  experi- 
ments throw  light  on  Skey's  experiments.  With  increasing 
relative  concentration  of  sulphuric  acid,  we  should  expect  an  in- 
creasing displacement  of  nitric  acid  from  the  charcoal,  which  is 
what  actually  occurred.  Lachs  and  Michaelis5  found  that  caus- 
tic potash  cut  down  very  much  the  adsorption  of  potassium  chlo- 
ride by  charcoal  from  water.  In  other  words,  the  presence  of 
the  strongly  adsorbed  hydroxyl  ion  decreased  the  adsorption  of 
the  chloride  very  much.  They  also  found  that  if  sulphuric 
acid  is  added  in  small  amounts  to  the  chloride  solution 
the  adsorption  of  chloride  ion  is  increased  markedly.  This 
is  in  accord  with  the  results  of  Osaka,6  that  potassium 
chloride  is  adsorbed  more  than  potassium  sulphate,  or  sodium 
sulphate,  and,  presumably,  hydrochloric  acid  more  than  sul- 
phuric acid. 

Thorium  salts  cut  down  the  adsorption  of  uranium  X  by  char- 
coal7 and  acetone  and  acetic  acid  decrease  the  adsorption  of 
grape-sugar  by  charcoal8  but  albumin  and  acetone  have  no  effect 

1  Chem.  News,  17,  217  (1886). 

2  Jour.  Am.  Chem.  Soc.,  30,  1784  (1908). 

3  Zeit.  phys.  Chem.,  74,  730  (1910). 

4  Van  Bemmelen  Gedenkboek,  88  (1910). 
6  Zeit.  Elektrochemie,  17,  1  (1911). 

6  Mem.  Coll.  Sci.  Kyoto  Univ.,   1,  267  (1915). 

7  FREUNDLICH  and  KAEMPFER:  Zeit.  phys.  Chem.,  90,  681  (1915). 

8  RON  A  and  MICHAELIS:  Biochem.  Zeit.,  16,  499  (1909). 


ADSORPTION  FROM  SOLUTION 


115 


on  each  other,  nor  does  one  acid  dye  apparently  displace  another 
on  wool  or  silk.1 

If  we  have  two  salts  with  no  common  ion,  it  is  easy  to  see  that 
the  most  strongly  adsorbed  cation  and  anion  will  be  taken  up  the 
most,  while  the  other  pair  will  be  adsorbed  the  least  readily. 
In  the  case  of  dyeing  we  have  an  admirable  illustration  of  the 
fact,  first  recognized  by  Lachs  and  Michaelis,2  and  by  Estrup,3 
that  an  anion  is  adsorbed  more  readily  in  presence  of  a  readily 
adsorbed  cation  and  a  cation  in  presence  of  a  readily  adsorbed 
anion.  In  an  acid  dye  the  color  is  in  the  acid  radical  and  in  a 
basic  dye  the  color  is  in  the  basic  radical.  An  acid  dye  will 
therefore  be  taken  up  more  strongly  in  an  acid  solution  than  in 
a  neutral  solution  and  will  be  taken  up  least  in  an  alkaline  solu- 
tion. A  readily  adsorbed  anion  will  decrease  the  amount  of 
dye  taken  up  and  a  readily  adsorbed  cation  will  increase  it. 
With  a  basic  dye  the  reverse  will  be  true.  The  dye  will  be  taken 
up  most  readily  in  an  alkaline  solution  but  may  be  taken  up  in 
a  natural  or  acid  solution.  A  readily  adsorbed  cation  will  cut 
down  the  adsorption  of  the  dye  and  a  readily  adsorbed  anion 
will  increase  it. 

In  Tables  XXXI-XXXII  are  data  by  Pelet-Jolivet4  for  crystal 
ponceau,  an  acid  dye. 

TABLE  XXXI. — ADSORPTION  OF  CRYSTAL  PONCEAU  BY  WOOL 

Volume  of  solution  =  200  cc. 
Weight  of  wool  =  5  grams. 
x  =  percent  crystal  ponceau. 


Milligrams  crystal  ponceau  adsorbed 

3  cc  N/10  H2S04 

Water 

0.12  g  NazSOi 

0.1 

65 

41.7 

36.2 

0.2 

113 

62.8 

59.8 

0.3 

150 

76.5 

64.8 

0.4 

178 

86.5 

70.8 

*LAKE:  Jour.  Phys.  Chem.,  20,  751  (1916). 
2Zeit.  Elektrochemie,  17,  1  (1911). 

3  Zeit.  Kolloidchemie,  11,  8  (1912). 

4  Die  Theorie  des  Farbeprozesses,  94,  149  (1910). 


116  APPLIED  COLLOID  CHEMISTRY 

TABLE  XXXII. — ADSORPTION  OF  CRYSTAL  PONCEAU  BY  WOOL 

Volume  of  solution  =  200  cc. 
Weight  of  wool  =  5  grams. 
Crystal  ponceau  =  0.372  mg. 


Milligrams  crystal  ponceau  adsorbed 

cc  N/10  acid 

added 

HC1 

H2SO4 

H3P04 

0 

44 

42 

42 

8 

109 

93 

48 

18 

158 

140 

80 

28 

171 

153 

102 

In  acid  solution  crystal  ponceau  is  adsorbed  more  strongly 
than  in  a  water  solution,  while  the  presence  of  the  strongly  ad- 
sorbed anion  of  sodium  sulphate  cuts  down  the  adsorption  over 
that  in  water.  In  the  acid  solution  the  strongly  adsorbed  phos- 
phate anion  works  against  the  strongly  adsorbed  hydrogen  cation 
and  consequently  the  dye  is  least  adsorbed  in  a  phosphoric  acid 
solution  and  most  adsorbed  in  a  hydrochloric  acid  solution. 

In  Tables  XXXIII  and  XXXIV  are  data  by  Pelet-Jolivet1 

TABLE  XXXIII. — ADSORPTION  OF  METHYLENE  BLUE  BY  SILK  AND  COTTON 

A  =  0.3  gram  silk  in  50  cc  0.2  percent  methylene  blue. 
B  =  3  grams  cotton  in  100  cc  0.1  percent  methylene  blue. 


Fiber 

Methylene  blue  adsorbed,  mg 

2  cc  N/10  HC1 

Neutral  solution 

2  cc  N/10  KOH 

Silk   (A) 

1.3 
1.2 

10.8 
11.2 

40.6 

30.0 

Cotton  (B)  

for  methylene  blue,  a  basic  dye.  The  most  dye  is  taken  up  in  the 
alkaline  solution  and  the  least  in  the  acid  solution  which  is  just 
as  it  should  be.  For  the  same  concentration  of  acid,  more  dye 
is  taken  up  in  a  phosphate  solution  than  in  a  sulphate  solution 
while  still  less  is  adsorbed  from  a  hydrochloric  acid  solution; 
and  we  should  expect  the  phosphate  radical  to  be  adsorbed  the 
iDieTheorie  des  Farbeprozesses,  95  (1910). 


ADSORPTION  FROM  SOLUTION 


117 


most  and  the  chlorine  radical  the  least.  This  comparison  is 
not  strictly  accurate  because  equivalent  concentrations  of  two 
acids  do  not  necessarily  mean  equal  degrees  of  acidity.  On  the 
other  hand  the  amount  of  hydrogen  as  ion  in  20  cc  N/10  H2SO4 

TABLE  XXXIV. — ADSORPTION  OF  METHYLENE  BLUE  BY  WOOL 

Volume  of  solution  =  200  cc. 

Weight  of  wool  =  3  grams. 

Concentration  of  methylene  blue  =0.8  percent. 


Methylene  blue  adsorbed,  mg 

N/10  HC1 

N/10  H2S04 

N/10  HjP04 

0 

156 

156 

152 

10 

22 

37 

79 

20 

20 

27 

32 

30 

17 

is  unquestionably  greater  than  in  10  cc  N/10HC1  after  both  have 
been  diluted  to  200  cc,  and,  yet,  more  methylene  blue  is  taken 
up  in  this  sulphuric  acid  solution  than  in  the  hydrochloric  acid 
solution.  The  difficulty  in  regard  to  the  acidity  can  be  got  round 
by  adding  salts.  Some  data1  are  given  in  Table  XXXV.  At 

TABLE  XXXV. — ADSORPTION  OF  METHYLENE  BLUE  BY  WOOL 

Volume  of  solution  =  100  cc. 
Weight  of  wool  =  1  gram. 


Methylene  blue  adsorbed,  mg 


Methylene  blue  in 

solution,  percent 

No  salt 

0.06  g  Na"SO4 

0.06  g  NazHPO« 

0.1 

35 

45.6 

81.1 

0.2 

40 

52.2 

91.3 

0.3 

43 

56.4 

95.6 

0.4 

44 

59.4 

98.6 

every  concentration  of  dye  more  dye  is  fixed  in  the  bath  contain- 
ing sulphate  than  in  the  bath  to  which  no  salt  is  added,  while 
still  more  dye  is  adsorbed  in  the  phosphate  solution.     \Ve  see 
1  Pelet-Jolivet:  Die  Theorie  des  Farbeprozesses,  118  (.1910). 


118  APPLIED  COLLOID  CHEMISTRY 

thus  the  amount  of  adsorption  varying  in  the  order  Na2HPO4> 
Na2SO4  >  NaCl  >  H2O.  Just  as  should  be  the  case,  sodium  sul- 
phate increases  the  amount  of  a  basic  dye  taken  up,  whereas 
it  decreases  the  amount  of  an  acid  dye  adsorbed.  With  0.5 
gram  silk  in  50  cc  0.2  percent  methylene  blue  solution,  the 
amounts  of  methylene  blue  taken  up  were  28.6,  18.4,  13.3,  and 
10.8  mg  when  sodium  phosphate,  sodium  sulphate,  sodium  chlo- 
ride, and  nothing  were  added  respectively  to  the  solution.1 
With  cotton  there  was  no  difference  under  the  same  circum- 
stances; but  there  was  a  marked  difference  when  the  cotton  had 
been  treated  with  tannin.  The  effect  of  different  cations  was 
also  studied  by  Pelet-Jolivet  who  found  that  silk  took  up  13.3 
mg  methylene  blue  from  a  solution  containing  sodium  chloride 
and  only  6.3  mg  from  a  solution  containing  barium  chloride,  the 
strongly  adsorbed  barium  cation  thus  cutting  down  the  adsorp- 
tion of  the  basic  dye.  With  wool  the  depth  of  color  decreases 
in  the  following  order:  sodium  phosphate,  sodium  sulphate,  no 
addition,  barium  chloride,  and  platinum  tetrachloride.  Cotton 
takes  up  less  methylene  blue  from  a  solution  containing  barium 
chloride  than  from  one  containing  sodium  chloride. 

With  substantive  dyes  the  effects  of  electrolytes  are  different 
because  these  dyes  are  in  colloidal  solution  and  the  question  of 
stability  enters  in.  Abnormal  results  are  also  to  be  expected 
with  acid  and  basic  dyes  which  form  colloidal  solutions. 

When  an  acid  dye  is  adsorbed  nearly  completely  by  the  fiber  it 
may  easily  happen  that  the  differences  between  acids  becomes  so 
small  as  to  be  practically  negligible.2  The  concentration  of  the 
anion  can  be  increased  very  much  relatively  to  the  concentration 
of  hydrogen  as  ion  by  adding  salt,  and  thus  the  effect  of  the  anion 
can  be  brought  out.  This  is  shown  in  Table  XXXVI.  In  the 
third  column  are  given  the  amounts  of  dye  adsorbed  when  acids 
alone  are  added  to  the  bath.  In  the  fifth  column  are  the  amounts 
of  dye  adsorbed  when  the  solution  contains  10  cc  N/10  acid 
+  10  cc  N/l  salt.  With  the  acids  alone  the  amount  of  adsorption 
of  acid  violet  is  practically  independent  of  the  nature  of  the  acid. 
With  croceine  orange,  sulphuric  acid  and  phosphoric  acid  behave 
practically  alike  and  cut  down  the  adsorption  as  compared  with 

1  PELET-JOLIVET:  Die  Theorie  des  Farbeprozesses,  118  (1010). 

2  LAKE:  Jour.  Phys.  Chem.,  20,  785  (1916). 


ADSORPTION  FROM  SOLUTION 


119 


hydrochloric  acid.  With  acid  violet  the  addition  of  sodium 
chloride  has  no  appreciable  effect.  The  slight  apparent  increase 
in  adsorption  is  probably  due  to  experimental  error.  With 
croceine  orange  the  sodium  chloride  cuts  down  the  adsorption 
slightly.  With  sodium  phosphate  the  effect  is  quite  extraordinary, 

TABLE  XXXVI. — ADSORPTION  OF  ACID  VIOLET  AND  CROCEINE  ORANGE  BY 

WOOL 

Volum^  of  solution  =  250  cc. 
Weight  of  wool  =  1  gm. 
Amount  of  dye  =  40  mg. 
Temperature  =  100°. 
Time  of  run  =  1  hr. 


Acid  dye 

10  cc  N/10 
acid 

Adsorbed 
dye,   mg 

Acid  +  10 
cc  N/l  salt 

Adsorbed 
dye,  mg 

Acid  violet  

HC1 

39.2 

NaCl 

39.5 

Croceine  orange. 

H2S04 
H3P04 
HC1 

39.2 
39.1 
38.0 

Na2S04 
Na2HPO4 
NaCl 

36.8 
l.O1 
33.4 

H2SO4 
H3P04 

35.5 
35.5 

Na2S04 
Na2HPO4 

20.0 
O.I1 

the  adsorption  of  acid  violet  and  of  croceine  orange  being  cut 
almost  to  zero.  The  change  in  the  hydrogen  ion  concentration 
cannot  account  for  this  because  38  mg  acid  violet  and  32  mg 
croceine  orange  are  adsorbed  from  a  water  solution  under  similar 
circumstances.  We  are  therefore  dealing  with  a  specific  effect 
of  the  phosphate  radical. 

Qualitative  experiments  along  the  same  lines  have  been  ob- 
tained by  Davison.2  With  acid  green  and  emerald  green  on 
wool  and  with  fast  green  and  safranine  on  cotton,  good  illustra- 
tions are  obtained  of  the  generalization  that  acid  dyes  are  taken 
up  more  completely  in  an  acid  solution  than  in  a  neutral  one 
and  least  of  all  in  an  alkaline  solution,  while  the  reverse  is  true 
with  basic  dyes.  Marked  differences  between  the  dyeing  in 
acid  and  alkaline  solutions  can  be  obtained  on  wool  with  fast 
green,  fast  blue,  acid  violet,  croceine  orange,  crystal  ponceau, 
chrysoidine,  safranine,  and  methylene  blue;  on  cotton  with 

1  These  experiments  are  probably  not  strictly  comparable  with  the  others 
owing  to  the  change  in  hydrogen  ion  concentration. 

2  Jour.  Phys.  Chem.,  17,  737  (1913). 


120  APPLIED  COLLOID  CHEMISTRY 

acid  violet,  croceine  orange,  and  emerald  green.  The  generaliza- 
tion that  addition  of  sodium  sulphate  cuts  down  the  adsorption 
of  acid  dyes  and  increases  the  adsorption  of  basic  dyes  is  shown 
excellently  on  wool  with  acid  green,  fast  blue,  croceine  orange, 
Victoria  blue,  chrysoidine,  and  safranine;  on  cotton  with  benzo- 
purpurine.  The  effect  of  continuously  varying  concentrations 
of  sodium  sulphate  is  shown  well  on  wool  with  brilliant  blue  and 
safranine;  on  cotton  with  safranine.  When  alumina  is  precipi- 
tated from  an  aluminum  acetate  solution,  it  adsorbs  fast  green, 
acid  green,  and  croceine  orange  considerably;  but  when  precipi- 
tated from  an  aluminum  sulphate  solution  it  carries  down  practi- 
cally no  fast  green,  acid  green,  or  croceine  orange.  The  presence 
of  sulphate  decreases  the  adsorption  of  acid  dyes  by  alumina 
as  well  as  by  wool.  Another  illustration  of  this  is  that  alumina 
precipitated  from  an  aluminum  sulphate  solution  can  only 
decolorize  dilute  solutions  of  alizarine,  alizarine  green,  benzo 
orange,  coeruleine,  and  acid  violet,  while  alumina  precipitated 
from  an  acetate  solution  will  decolorize  more  concentrated  solu- 
tions. In  these  experiments  with  alumina  the  dye  was  added  to 
the  solution  before  the  alumina  was  precipitated.  It  seems 
probable  that  if  alumina  were  precipitated  from  a  sulphate 
solution  with  barium  hydroxide  or  calcium  hydroxide,  the  effect 
due  to  the  sulphate  would  disappear  owing  to  the  insolubility  of 
the  barium  or  calcium  sulphate.  This  experiment  was  not  tried. 

CHEMICAL  ACTION 

Selective  adsorption  connotes  the  possibility  of  chemical 
decomposition  as  a  result  of  adsorption.  If  a  given  substance 
adsorbs  a  base  more  strongly  than  an  acid  for  instance,  there 
will  be  a  tendency  for  the  salt  of  that  base  and  that  acid  to  hydro- 
lyze,  the  base  being  then  adsorbed  to  a  greater  extent  than  the 
acid.  Theoretically  there  is  always  some  hydrolysis  even  with 
sodium  chloride  and  water  according  to  the  equation 

Na'  +  Cl'  +  H20  =  Na-  +  OIF  +  IT  +  Cl' 
but  this  reaction  does  not  run  far  because  caustic  soda  and 
hydrochloric  acid  are  strong  electrolytes  and  we  cannot  have  a 
high  simultaneous  concentration  of  hydrogen  and  hydroxyl  as 
ions.  If  the  caustic  soda  is  removed  by  adsorption,  the  hydrol- 
ysis can  go  farther.  How  complete  the  hydrolysis  will  be 


ADSORPTION  FROM  SOLUTION  121 

depends  on  the  degree  to  which  the  base  is  adsorbed  and  on 
the  strength  of  the  acid.  Any  acid  will  tend  to  react  with 
adsorbed  caustic  soda;  but  that  tendency  will  be  greater  the 
stronger  the  acid.  We  thus  see  that  a  neutral  solution  will 
become  acid  if  shaken  with  a  substance  which  adsorbs  the  base 
much  more  strongly  than  the  acid,  that  it  will  become  alkaline  if 
the  substance  adsorbs  the  acid  much  more  strongly  than  the 
base,  and  that  it  will  remain  neutral  in  case  the  solid  does  not 
adsorb  either  base  or  acid  at  all,  or  in  case  it  adsorbs  the  two  in 
practically  equivalent  amounts  either  separately  or  as  undissoci- 
ated  salt.  The  amount  of  base  or  of  acid  taken  up  from  a  salt 
solution  will  be  greater  the  weaker  the  unadsorbed  acid  or  base. 
Many  cases  are  known  where  the  solution  does  become  acid  or 
alkaline  when  shaken  with  an  adsorbing  agent  and  this  has 
nothing  to  do  with  any  hypothetical  acidity  or  alkalinity  of  the 
adsorbing  substance. 

If  fuller's  earth  be  shaken  with  water  and  then  filtered,  the 
filtrate  is  neutral  to  litmus  paper1  or  to  phenolphthalein,  showing 
that  no  soluble  base  or  acid  is  present.  If  fuller's  earth  be 
shaken  with  a  sodium  chloride  solution  and  filtered,  the  filtrate 
is  acid  to  litmus  or  to  phenolphthalein.  This  is  because  fuller's 
earth  has  adsorbed  the  base.  If  one  presses  litmus  paper  against 
moistened  fuller's  earth,  the  litmus  paper  turns  red,  and  if  one 
adds  fuller's  earth  to  a  faintly  alkaline  solution  of  phenolphtha- 
lein, the  red  color  disappears.  This  is  not  because  the  fuller's 
earth  is  acid,  but  because  it  takes  the  base  from  the  sodium  chlo- 
ride, the  litmus,  or  the  phenolphthalein. 

I  have  been  told  that  the  adsorbing  power  of  fuller's  earth  is  so 
great  that  an  acre-foot  as  soil  would  adsorb  30,000  Ib.  of  lime 
and  that  this  would  make  fuller's  earth  about  equivalent  in 
acidity  to  a  two  percent  sulphuric  acid  solution. 

Silk  has  such  a  marked  selective  adsorption  for  rosaniline 
(magenta)  that  it  will  not  only  decompose  the  hydrochloride, 
leaving  the  acid  behind,  but  will  also  convert  the  carbinol  base 
back  into  the  color  base  in  presence  of  an  excess  of  ammonia,2 
the  silk  being  dyed  red  from  a  colorless  ammoniacal  solution. 

1  Cf.  CAMERON:  Jour.  Phys.  Chem.,  14,  400  (.1910). 
2jACQUEMiN:  Comptes  rendus,  82,  261   (1876);  see  also  MILLS:  Jour. 
Chem.  Soc.,  35,  27  (1879);  FORTUYN:  Zeit.  phys.  Chem.,  90,  236  (1915.) 


122  APPLIED  COLLOID  CHEMISTRY 

This  is  the  more  remarkable  because  the  free  color  base  is  in- 
stable  and  cannot  be  isolated  in  a  pure  state.  This  makes  it 
seem  certain  that  silk  stabilizes  the  free  color  base.  Another 
similar  instance  is  the  mordanting  of  wool  with  copper  salts, 
coloring  it  green.  This  green  does  not  change  to  black  when  the 
wool  is  heated  to  boiling,  though  hydrous  copper  oxide  by  itself 
changes  very  readily.  Blucher  and  Farnau1  have  extended 
Tommasi's2  experiments  on  the  stabilization  of  hydrous  copper 
oxide  by  manganese  salts  and  find  that  a  number  of  other  metallic 
hydroxides  are  also  effective.  Bayliss3  has  obtained  results, 
which  indicate  that  alumina  stabilizes  the  free  acid  of  Congo  red, 
and  Schaposchnikoff  and  Bogojawlenski4  have  isolated  this 
metastable  form  by  allowing  the  pyridine  salt  to  effloresce.  Pas- 
sive iron  is  an  instable  oxide,  presumably  Fe02  or  Fe03,  stabilized 
when  adsorbed  by  metallic  iron.5 

When  discussing  the  adsorption  of  gases  by  solids,  numerous 
instances  were  given  of  so-called  contact  catalysis.  With  solu- 
tions the  effect  of  contact  materials  is  as  yet  relatively  unim- 
portant. Lassar-Cohn6  gives  a  few  instances  where  yields  have 
been  increased  by  the  use  of  porous  masses.  When  two  parts  of 
acetanilide  were  heated  with  one  part  of  zinc  chloride,  a  yield  of 
3.5  percent  flavaniline  hydrochloride  was  obtained.  In  presente 
of  two  parts  of  infusorial  earth  the  yield  increased  to  about  7.5 
percent.  When  benzene  and  chloroform  react  in  presence  of 
aluminum  bromide,  the  yield  increases  from  33  percent  *to  44 
percent  when  infusorial  earth  is  added.  The  sulphonation  of 
benzene,  xylene,  naphthalene,  anthraquinone,  aniline,  etc.,  is 
simplified  very  much  by  the  addition  of  infusorial  earth  or  of 
charcoal.  When  a-dinitronaphthalene  is  treated  with  sulphuric 
acid  under  suitable  conditions  in  presence  of  infusorial  earth  or 
charcoal,  oxidation  takes  place  with  elimination  of  the  nitro 
groups  and  formation  of  naphthazarin. 

1  Jour.  Phys.  Chem.,  18,  629  (1914). 

2  TOMMASI:  Bull.  Soc.  chim.  Paris  (2)  37,  197  (1882);  Comptes  rendus 
99,  37  (1884). 

3  Proc.  Roy.  Soc.,  84B,  881  (1911). 

4  Jour.  Russ.  Phys.  Chem.  Soc.  44,  1813  (1913).' 

6  BENNETT  and  BURNHAM:  Jour.  Phys.  Chem.,  21,  107  (1917). 
6  Arbeitsmethoden   fiir  organisch-chemische  Laboratorien,  584,  72,  1079, 
923  (1903). 


ADSORPTION  FROM  SOLUTION  123 

Gurwitsch1  reports  that  amylene  polymerizes  readily  on  stand- 
ing in  contact  with  "  floridin,"  which  is  a  hydrous  silicate  of  some 
sort.  Playfair2  found  that  a  solution  of  nitric  acid  could  be  pre- 
pared which  would  only  bleach  a  piece  of  cloth  dyed  with  indigo 
when  the  cloth  was  in  contact  with  alumina,  chromic  oxide,  or 
stannic  oxide.  Richardson3  believes  that  petroleum  and  bitumens 
are  formed  by  the  polymerizing  or  condensing  action  of  clays  or 
sands. 

A  case  which  has  been  studied  a  great  deal  quantitatively 
is  the  decomposition  of  hydrogen  peroxide  solutions  by  platinum. 
One  of  the  most  interesting  things  about  this  reaction  is  its  ex- 
treme sensitiveness  to  so-called  poisons.4  The  rate  of  decomposi- 
tion of  hydrogen  peroxide  by  a  given  solution  of  colloidal  plati- 
num was  reduced  approximately  to  one-half  by  M/20,000,000 
HCN,  M/2,000,000  HgCl2,  and  M/300,000  H2S.  Curiously 
enough  the  catalytic  action  of  platinum  and  of  ferments  on  hydro- 
gen peroxide  is  decreased  in  both  cases  by  very  slight  traces  of 
poisons,  such  as  prussic  acid,  hydrogen  sulphide,  etc.  It  has 
also  been  shown  by  Schonbein5  that  these  same  substances  cut 
down  the  catalytic  action  of  red  blood  corpuscles  on  hydrogen 
peroxide. 

In  Table  XXXVII  are  given  data6  for  the  concentrations  neces- 
sary to  cut  down  the  rate  of  decomposition  of  hydrogen  peroxide 
to  one-half  in  the  case  of  colloidal  platinum  and  of  haemase,  the 
active  enzyme  in  the  red  blood  corpuscles.  While  many  of  the 
substances  that  cut  down  the  platinum  catalysis  also  decrease 
the  catalytic  action  of  haemase,  there  are  many  cases  where  no 
parallelism  occurs.  Thus  carbon  monoxide  is  very  toxic  to  plati- 
num and  has  no  effect  on  haemase.  On  the  other  hand,  nitric 
acid,  sulphuric  acid,  potassium  nitrate,  and  potassium  chlorate 
have  practically  no  effect  on  platinum  and  are  quite  toxic  to 
haemase.  Even  where  there  is  a  general  parallelism,  one  must 
not  follow  it  too  closely.  ,  With  platinum  the  prussic  acid  solution 

1  Zeit.  Kolloidchemie,  11,  18  (1912). 

2  Mem.  Chem.,  Soc.  3,  354  (1847). 

3  Jour.  Ind.  Eng.  Chem.,  8,  4  (1916);  Met.  Chem.  Eng.,  16,  25  (1917). 

4  BKEDIQ  and  VON  BERNECK:  Zeit.  phys.  Chem.,  31,  258  (1899);  BREDIQ 
and  IKEDA:  Ibid.,  37,  1  (1901). 

5  Jour,  prakt.  Chem.,  105,  202  (1868). 
CENTER:  Zeit.  phys.  Chem.,  61,  701  (1905). 


124 


APPLIED  COLLOID  CHEMISTRY 


has  one-tenth  the  concentration  of  the  mercuric  chloride  solution, 
while  with  haemase  it  has  double  the  concentration.  The  tabu- 
lated concentration  for  mercuric  chloride  is  the  same  for  platinum 
as  for  haemase,  while  that  of  iodine  dissolved  in  potassium  iodide 
is  one  hundred  times  as  great  for  haemase.  Kastle  and  Loeven- 
hart1  point  out  that  prussic  acid  is  a  strong  poison  for  colloidal 

TABLE  XXXVII. — THE  POISONING  OF  COLLOIDAL  PLATINUM  AND  HAEMASE 

Concentrations  at  which  poisons  reduce  the  rate  of  the  catalytic  decom- 
position of  hydrogen  peroxide  approximately  to  one-half. 


Poison 

Colloidal  platinum 

Haemase 

H2S 

M  /300,000 

M  /  1,000,  000 

HCN  

M  /20,000,  000 

M/1,000,000 

HgCl2 

M  /2,  000,  000 

M  /2,  000,  000 

He-Bra 

M/300  000 

Hg(CN)2  

M  /200,  000 

M  /300,  000 

I2  in  KI 

M/5,000,000 

M  /50,t)00 

NH2OH.HC1  

M  /25,  000 

M  /80,  000 

Phenyl  hydrazine   . 

M  /20,  000 

Aniline 

M  /5,  000 

M/400 

Arsenious  acid  
CO 

M/50 

Very  poisonous 

No  poisoning  at  M/2,000 
No  poisoning 

HC1  1  

M  /3,  000 

M/  100,000 

NH4C1  
HNO3 

M/200 
No  poisoning 

M/1,000 
M  /250,000 

H2SO4  •  
KNO3  
KC1O3  

No  poisoning 
No  poisoning 
Slight  poisoning? 

M/50,000 

M/40,000  at  0° 
M  /40,000  at  0° 

platinum  and  silver  but  accelerates  the  catalysis  of  hydrogen 
peroxide  by  iron  and  copper. 

It  seems  probable  that  the  poisons  are  adsorbed  strongly  by  the 
catalytic  agent  and  therefore  prevent  the  adsorption  of  hydrogen 
peroxide.2  This  adsorption  will  be  specific  and  will  therefore 
in  general  not  be  the  same  quantitatively  for  different  catalytic 
agents.  On  the  other  hand,  there  may  well  be  a  qualitative 
agreement  in  some  or  even  in  many  cases.  Owing  to  the  extreme 

1  Am.  Chem.  Jour.,  .29,  397  (1903). 

2  Cf.  SENTEB:  Zeit.  phys.  Chem.,  61,  702  (1905);  DENHAM:  Ibid.,  72,  689 
(1910). 


ADSORPTION  FROM  SOLUTION  125 

flexibility  of  this  hypothesis  there  is  nothing  as  yet  to  prevent  its 
accounting  for  all  the  phenomena  observed.1  On  the  other  hand 
it  must  be  remembered  that  proof  is  still  lacking  that  platinum 
adsorbs  prussic  acid  for  instance,  very  much  more  strongly  than 
hydrogen  sulphide,  and  that  there  is  no  difference  with  hae- 
mase.  There  may  be  other  factors  which  must  be  taken  into 
account.  Thus  Bredig2  points  out  that  when  colloidal  platinum 
is  allowed  to  stand  in  contact  with  hydrogen  peroxide  and  con- 
centrated potassium  cyanide,  the  platinum  flocculates  and  pre- 
cipitates. The  agglomerated  platinum  causes  the  hydrogen 
peroxide  to  decompose,  thus  showing  that  the  cyanide  does  not 
poison  precipitated  platinum  black.  There  seem  to  be  only  two 
possible  explanations.  One  is  that  the  adsorption  of  potassium 
cyanide  by  platinum  falls  off  very  much  more  rapidly  with  in- 
creasing size  of  the  platinum  particles  than  the  adsorption  of 
hydrogen  peroxide  by  platinum.  The  other  explanation  is  that, 
through  oxidation  or  otherwise,  there  is  formed  what  might  be 
called  an  anti-body,  which  cuts  down  the  adsorption  of  the  cya- 
nide. Neither  hypothesis  is  very  satisfactory  and  there  is  no 
experimental  evidence  for  either,  except  that  Jablczynski3 
found  that  platinized  platinum  decomposes  chromous  chloride 
three  times  as  fast  and  hydrogen  peroxide  one  hundred  times  as 
fast  as  does  smooth  platinum  foil.  The  matter  should  be  taken 
up  because  Rosnyak4  claims  that  the  catalytic  action  of  platinum 
on  hydrogen  peroxide  decreases  with  increasing  dispersity  of 
the  platinum  and  Felgate5  states  that  pulverulent  nickel  reduces 
nitric  oxide  while  colloidal  nickel  does  not.  In  both  these 
cases  it  is  probable  that  some  factor  has  been  overlooked  or  not 
stated.  The  more  finely  divided  the  platinum  the  more  likely 
the  surface  is  to  be  coated  with  oxide  and  it  does  not  follow  at  all 
that  the  catalytic  action  of  metallic  platinum  is  the  same  as  that 
of  an  oxide  of  platinum.  If  platinum  is  kept  in  suspension  by 
means  of  gelatine  or  some  similar  substances,  this  will  have  an 
effect  on  the  adsorption  and  the  catalytic  action. 

1  Cf .  FREUNDLICH  and  KAEMPFER:  Zeit.  phys.  Chem.,  90,  681  (1915). 

2  Zeit.  phys.  Chem.,  31,  332  (1899). 
8  Ibid.  64,  751  (1908). 

«Ibid.  85,68  (1913). 

6  Chem.  News,  108,  178  (1913). 


126  APPLIED  COLLOID  CHEMISTRY 

A  comparatively  recent  technical  development  in  contact 
catalysis  is  the  hardening  of  oils,  by  the  conversion  of  oleic  acid 
or  olein,  for  instance,  into  stearic  acid  or  stearin  by  the  action  of 
hydrogen  on  the  liquid  unsaturated  oil  in  presence  of  a  suitable 
catalytic  agent.1  In  1908  Paal2  proposed  using  a  colloidal  metal 
as  the  catalytic  agent  at  ordinary  temperatures.  Colloidal 
solutions  of  platinum,  palladium,  osmium,  and  iridium  have  been 
made,  of  which  the  palladium  hydrosol  appears  to  possess  the 
greatest  value.  When  hydrogen  at  ordinary  temperatures  is 
passed  into  glycerides  of  the  unsaturated  fatty  acid  to  which  some 
palladium  hydrosol  has  been  added,  reduction  takes  place.  It 
is  more  satisfactory,  however,  to  precipitate  the  metal  on  an 
inert  base  such  as  magnesia  or  kieselguhr. 

A  cheaper  and  apparently  better  method  is  to  use  nickel  as  a 
catalytic  agent  for  the  hydrogenation  of  liquid,  unsaturated 
acids,  the  reduction  taking  place  at  about  250°-300°.  There  has 
been  some  controversy  whether  the  catalytic  agent  was  really 
metallic  nickel3  or  an  unspecified  oxide  or  oxyhydride  ;4  but  there 
seems  to  be  little  doubt  but  that  the  metal  is  the  catalytic  agent. 
Some  details  in  regard  to  the  preparation  of  the  catalytic  agent 
are  given  by  Ellis.5  Paal  and  Karl6  report  that  the  oxides  of  lead, 
cadmium,  zinc,  aluminum  and  iron  cut  down  the  catalytic  action 
of  palladium,  while  the  oxides  of  magnesium,  nickel,  and  co- 
balt do  not. 

ADSORPTION  FROM  SOLUTION  BY  LIQUID 

There  is  no  reason  why  there  should  not  be  adsorption  by  a 
liquid  just  as  well  as  by  a  solid,  except  that  the  matter  is  com- 
plicated by  the  possibility  of  the  solute  dissolving  in  the  second 

1  JOBLING:  Catalysis  and  its  Industrial  Applications,  65,  75  (1916). 

2  Ber.  deutsch.  chem.  Ges.,  41,  2282  (1908). 

3  MEIGEN:  Jour,  prakt.  Chem.  (2)  92,  390  (1915). 

4  BEDFORD  and  ERDMANN:  Jour,  prakt.  Chem.  (2)  87,  425  (1913);  SIEG- 
MUND  and  SAIDA:  Ibid.  (2)  91,  442  (1915);  ERDMANN:  Ibid.  (2)  91,  469 
(1915);  GLASER:  Zeit.  anorg.  Chem.,  36,  1(1903);  BELLACCI  and  CORELLI: 
Ibid.,  86,  88  (1914);  SABATIER  and  ESPIL:  Comptes  rendus,  158,  568;  159, 
137  (1914). 

5  Jour.  Soc.  Chem.  Ind.,  31,  1163  (1912);    The  Hydrogenation  of  Oils, 
2d.  Ed.,  108  (1918). 

.     6  Ber.  deutsch.  chem.  Ges.,  46,  3069  (1913). 


ADSORPTION  FROM  SOLUTION  127 

liquid.  An  interesting  case  of  such  adsorption  was  studied 
by  Wilson,1  Swan,2  and  Twomey.3  When  a  drop  of  chloro- 
form or  carbon  tetrachloride  is  placed  on  the  bottom  of  a  glass 
dish  and  water  or  dilute  acid  poured  over  it,  the  drop  is  well 
rounded.  If  the  solution  is  made  alkaline,  the  drop  flattens  out. 
The  flattening  is  clearly  not  due  to  a  change  in  surface  tension  of 
the  water  phase,  because  Wilson  found  that  the  effect  of  alkali 
was  the  same  qualitatively  whether  sodium  chloride  was  present 
or  not,  whereas  the  addition  of  salt  increases  the  surface  tension 
of  the  water  phase.  What  happens  is  that  hydroxyl  is  adsorbed 
at  the  dineric  interface,  lowering  the  surface  tension  and  causing 
the  organic  liquid  to  flatten.  This  is  confirmed  by  the  experi- 
ments of  von  Lerch4  who  determined  the  surface  tension  between 
benzene  and  water  by  means  of  the  rise  in  capillary  tubes.5  The 
surface  tension  between  pure  benzene  and  pure  water  was  found 
to  be  32.6  dynes/cm  at  15°-16°.  With  M/4  NaOH  the  value 
dropped  to  20.7  and  with  M/2  NH4OH  to  27.3.  The  lowering 
of  the  surface  tension  is  greater  with  sodium  hydroxide  than  with 
ammonia  owing  to  the  greater  electrolytic  dissociation  of  the 
former.  The  surface  tension  was  also  lowered  by  hydrochloric 
acid  but  only  slightly,  M/2  HC1  bringing  it  down  to  31.5. 
This  is  in  accord  with  Wilson's  qualitative  conclusions  that  acids 
do  not  increase  the  surface  tension  quite  as  much  as  pure  water. 
Von  Lerch  also  found  a  marked  effect  due  to  acetic  acid;  but  this 
acid  is  miscible  in  all  proportions  with  benzene  and  causes  benzene 
and  water  to  become  more  miscible,  so  there  is  nothing  surprising 
about  its  behavior.  The  surface  tension  measurements  are  con- 
firmed by  the  electrical  ones,  for  von  Lerch  found  that  drops 
of  benzene  in  caustic  soda  moved  fairly  readily  under  electrical 
stress,  whereas  there  was  almost  no  movement  in  presence  of 
sodium  chloride  or  hydrochloric  acid.  It  is  possible  that  other 
ions  might  be  adsorbed  markedly  by  benzene,  but  I  know  of 
no  experiments  to  show  this.  Patrick6  found  that  new  fuchsine, 

1  Jour.  Chem.  Soc.,  1,  174  (1849). 

2  Phil.  Mag.  (3)  33,  36  (1848). 

3  Jour.  Phys.  Chem.  19,  360  (1915). 
*  Drude's  Ann.,  9,  434  (1902). 

6  HARKINS  states  that  VON  LERCH'S  experiments  with  dilute  caustic  soda 
solution  are  inaccurate. 

6  Zeit.  phys.  Chem.,  86,  545  (1914). 


128  APPLIED  COLLOID  CHEMISTRY 

picric  acid,  salicylic  acid,  and  mercurous  sulphate  are  adsorbed 
from  aqueous  solution  by  mercury,  the  surface  tension  of  the 
mercury  being  lowered  as  a  result  of  the  adsorption.  In  all 
four  cases  the  amounts  adsorbed  varied  with  the  concentration 
approximately  according  to  a  formula  of  the  type  found  to  hold 
for  adsorption  from  solution  by  a  solid. 

The  adsorption  of  hydroxyl  means  practically  a  concentrating 
of  alkali  at  the  surface  of  the  benzene  and  consequently  a  closely 
adhering  film  of  water.  This  accounts  for  Wilson's  observation 
that  the  drops  of  chloroform  were  readily  wetted  by  an  alkaline 
solution.  The  presence  of  the  surface  film  of  caustic  soda  solu- 
tion also  accounts  for  the  decreased  mobility  observed  by  Wilson. 


CHAPTER  IV 
SURFACE  TENSION— BROWNIAN  MOVEMENTS 

ADSORPTION  AND  SURFACE  TENSION 

When  a  substance  dissolves  in  a  liquid,  the  surface  tension 
between  the  liquid  and  the  vapor  changes,  increasing  in  some 
cases  and  decreasing  in  others.  Two  solutions  of  the  same  den- 
sity will  not  necessarily  have  the  same  surface  tension.  For 
this  reason  a  hydrometer  graduated  for  sulphuric  acid  solutions 
will  not  be  accurate  for  hydrochloric  acid  solutions,  because  the 
amounts  of  solution  which  will  rise  up  the  stem  of  the  hydro- 
meter will  depend  on  the  surface  tensions  and  will  therefore  not 
be  the  same  in  the  two  cases.  In  most  cases  the  surface  tensions 
of  solutions  will  lie  between  the  surface  tensions  of  the  constitu- 
ents; but  this  is  not  necessarily  true.1  Rontgen  and  Schneider2 
found  a  maximum  surface  tension  for  sulphuric  acid  and  water 
at  about  48  percent  H2SO4.  Whatmough3  obtained  minima 
for  mixtures  of  acetic  acid  with  benzene,  methyl  iodide,  ethyl 
iodide,  chloroform,  or  carbon  tetrachloride ;  for  carbon  bisulphide 
and  ethylene  chloride;  for  benzene  and  ethyl  iodide;  for  carbon 
tetrachloride  and  chloroform,  for  ethyl  acetate  and  amyl  alcohol. 
Most  salts  increase  the  surface  tension  of  the  solution,  the  in- 
crease being  very  nearly  proportional  to  the  concentration.  The 
strong  bases  increase  the  surface  tension  but  ammonia  lowers 
it  and  so  also  do  nitric,  hydrochloric,  and  hydrobromic  acid. 
Worley4  has  discovered  an  interesting  fact.  Although  sodium 
chloride  increases  the  surface  tension  of  water,  the  addition  of 
salt  to  a  solution  of  aniline  in  water  decreases  the  surface  tension. 
The  sodium  chloride  decreases  the  solubility  of  aniline  in  water 
and  therefore  increases  the  partial  pressure  of  the  aniline  vapor. 

1  See  FREUNDLICH  :  Kapillarchemie,  58  (1909). 

2  Wied.  Ann.,  29,  165  (1886). 
3Zeit.  phys.  Chem.,  39,  129  (1902). 

4  Jour.  Chem.  Soc.,  105,  260,  273  (1914). 
9  129 


APPLIED  COLLOID  CHEMISTRY 

The  vapor  pressures  therefore  correspond  to  a  solution  richer  in 
aniline  than  that  actually  present  and  the  surface  tension  relations 
run  approximately  parallel  with  the  vapor  pressures.  Worley 
noticed  another  curious  thing  in  regard  to  change  of  surface 
tensions  of  solutions  with  rising  temperature.  Since  liquid  and 
vapor  are  identical  at  the  critical  temperature,  the  surface  ten- 
sion becomes  zero  at  the  critical  point  and  the  surface  tension  of 
pure  liquids  therefore  decreases  with  rising  temperature.  One 
might  reasonably  have  expected  that  the  surface  tension  tem- 
perature curves  for  solutions  would  run  somewhere  near  parallel 
to  the  corresponding  curves  for  the  pure  constituents;  but  this  is 
not  the  case  for  mixtures  of  water  with  aniline  or  phenol.  The 
surface  tension  of  the  phenol-rich  solutions  vary  relatively  little 
with  rising  temperature,  while  the  corresponding  aniline  solu- 
tions actually  show  an  increase  of  surface  tension  with  rising 
temperature,  The  explanation  seems  to  be  that  the  ratio  of 
water  to  aniline  or  phenol  in  the  vapor  increases  with  rising 
temperature  and  that  the  solutions  therefore  show  surface 
tensions  corresponding  to  what  one  might  expect  of  solutions 
richer  in  water  than  they  actually  are.  It  is  a  pity  that  Worley 
did  not  also  study  some  such  case  as  ether  and  water,  where  the 
partially  miscible  liquid  with  the  lower  surface  tension  has  the 
higher  vapor  pressure. 

The  change  in  surface  tension  when  a  solution  is  formed  is  also 
accompanied  by  a  change  in  concentration,  the  surface  film  of  the 
liquid  having  a  different  concentration  from  the  mass  of  the  liquid 
solution.  The  simple  rule  in  regard  to  this  is  that  the  concentra- 
tion in  the  film  tends  to  change  so  as  to  decrease  the  surface 
tension.1  Consequently  the  surface  film  will  be  more  dilute 
than  the  mass  of  the  solution  if  the  dissolved  substance  increases 
the  surface  tension,  and  will  be  more  concentrated  than  it,  if  the 
solute  lowers  the  surface  tension  of  the  solution.  An  equilibrium 
will  be  reached  when  the  change  in  the  surface  tension  is  balanced 
by  the  difference  of  osmotic  pressure  between  the  surface  film 
and  the  mass  of  the  solution.  This  can  be  expressed  mathe- 
matically by  the  equation2 

da 
^~o> 

1  J.  WILLARD  GIBBS:  Scientific  Papers,  1,  219. 

2  Cf.  DONNAN  and  BARKER:  Proc.  Roy.  Soc.,  85A,  557  (1911). 


SURFACE  TENSION— BROWNIAN  MOVEMENTS         131 

where  a  is  the  interfacial  tension,  /i  is  the  chemical  potential 
of  the  dissolved  substance  in  the  aquoeus  phase,  y  is  the  mass 
of  solute  per  unit  area  of  interface  in  excess  of  that  corre- 
sponding to  the  uniform  bulk-concentration  of  the  solute  in 
the  solution.  It  will  be  seen  that  y  is,  in  fact,  the  amount  of 
solute  per  unit  area  of  interface  adsorbed  or  concentrated  in  the 
interfacial  transition  layer.  If  we  are  justified  in  applying  the 
laws  of  dilute  solutions,  the  above  equation  can  be  written 

c    da 
~RTdc 

where  R  is  the  constant  of  the  simple  gas  equation,  T  is  the  abso- 
lute thermodynamic  temperature  and  c  is  the  concentration. 
It  is  to  be  noted  that  these  two  equations  apply  only  to  the  case 
of  a  substance  which  is  really  in  solution,  a  point  which  is  apt  to 
be  overlooked  in  papers  on  colloids.  On  the  other  hand,  there 
are  many  substances  such  as  soap,  saponin,  gelatine,  which  are 
probably  not  soluble  in  water  to  any  appreciable  extent;  but 
which  form  apparent  or  colloidal  solutions  with  surface  tensions 
lower  than  those  of  pure  water.  In  these  cases  the  concentration 
of  the  added  substance  will  be  higher  in  the  surface  film  than  in 
the  mass  of  the  liquid,  but  the  Gibbs  formula  will  not  apply. 
As  a  matter  of  fact  with  sodium  glycocollate,  Congo  red, 
methyl  orange,  and  sodium  oleate,  the  adsorptions  in  the  surface 
are  20-100  times  those  calculated  from  the  formula.1 

If  the  difference  in  concentration  between  the  mass  of  the 
solution  and  the  surface  layer  is  set  up  slowly,  one  ought  to  get 
one  value  for  the  surface  tension  when  equilibrium  is  reached 
and  another  if  rapid  measurements  are  made  on  a  fresh  surface. 
This  has  been  done  by  Lord  Rayleigh,2  though  not  for  a  case  of 
true  solution.3  The  equilibrium  or  static  method  consisted  in 
measuring  the  rise  in  capillary  tubes.  The  rapid  or  dynamic 
method  depends  on  the  principle  that  a  jet  of  liquid  issuing  under 
moderate  pressure  from  an  elongated,  e.g.,  elliptical,  aperture 

1  WILLOWS  and  HATSCHEK:  Surface  Tension  and  Surface  Energy,   46 
(1915). 

2  Proc.  Roy.  Soc.,  47,  281  (1890). 

3  See  however  HABKINS  and  BROWN:  Jour.  Am.   Chem.  Soc.,  41,  522 
(1919). 


132 


APPLIED  COLLOID  CHEMISTRY 


perforated  in  a  thin  plate,  assumes  a  chain-like  appearance,  the 
complete  period,  X,  corresponding  to  two  links  of  the  chain,  being 
the  distance  traveled  over  by  a  given  part  of  the  liquid  in  the 
time  occupied  by  a  complete  transverse  vibration  of  the  column 
about  its  cylindrical  configuration  of  equilibrium.  Since  the 
phase  of  vibration  depends  upon  the  time  elapsed,  it  is  always 
the  same  at  the  same  point  in  space,  and  thus  the  motion  is 
steady  in  the  hydrodynamical  sense,  and  the  boundary  of  the  jet 
is  a  fixed  surface.  Measurements  of  X  under  a  given  head,  or 
velocity,  determine  the  time  of  vibration,  and  from  this,  when 
the  density  of  the  liquid  and  the  diameter  of  the  column  are 
known,  follows  in  its  turn  the  value  of  the  capillary  tension  to 
which  the  vibrations  are  due.  By  this  method  the  liquid  surface 
may  be  tested  before  it  is  Mooo  second  old. 

Rayleigh's  data  have  been  recalculated  by  Freundlich1  who 
added  some  of  his  own.     These  are  given  in  Table  XXXVIII. 

TABLE  XXXVIII. — STATIC  AND  DYNAMIC  SURFACE  TENSIONS  OF  SOLUTIONS 


Solution 

Concentration 

Surface  tensions  in  dynes/cm 

Static 

Dynamic 

Water  

75 
55 
26 
26 
26 
52 
54 

75 
79 
79 
62 
58 
73 
68 

Sodium  oleate 

0.025  percent 
0  .  25    percent 
1  .  25    percent 
2.5   .  percent 

Sodium  oleate  
Sodium  oleate. 

Sodium  oleate  

Saponin  

Heptylic  acid  

O.OOoN 

Except  in  the  case  of  pure  water  the  surface  tensions  by  the 
static  method  are  lower  than  those  by  the  dynamic  method.  One 
curious  point  is  that  the  surface  tensions  by  the  dynamic  method 
are  higher  than  that  of  pure  water  in  the  case  of  the  two  very  dilute 
sodium  oleate  solutions.  Lord  Rayleigh  did  not  discuss  this  point 
and  Freundlich  leaves  it  untouched.  In  the  actual  measure- 
ments an  error  of  about  two  percent  would  account  for  the  dif- 
ference, which  may  be  due  to  experimental  error.  There  is 
another  possibility,  however,  that  the  sodium  oleate  is  completely 

1  Kapillarchemie,  56  (1909). 


SURFACE  TENSION— BROWN1  AN  MOVEMENTS 


133 


hydrolyzed  in  the  dilute  solutions  and  that  what  is  measured  by 
the  dynamic  method  is  really  the  surface  tension  of  a  caustic 
soda  solution. 

While  this  set  of  experiments  gives  more  or  less  the  results  that 
one  would  expect,  it  seems  impossible  to  account  for  the  difference 
between  the  static  and  dynamic  methods  in  the  case  of  the  ad- 
sorption of  gases  by  mercury.  Stockle1  measured  the  surface 
tension  of  mercury  in  a  vacuum  of  0.004-0.001  mm  and  ob- 
tained a  value  of  436  dynes/ cm  at  15°  regardless  whether  the 
measurement  was  made  at  once  or  not.  When  hydrogen  was 
present,  the  time  factor  was  important,  a  value  of  470  dynes/ cm 
being  obtained  when  the  measurement  was  made  quickly,  but 
dropping  to  434  dynes/cm  if  the  mercury  were  allowed  to  stand 
in  contact  with  hydrogen.  Similar  results  were  obtained  with 
oxygen,  nitrogen,  carbon  dioxide,  and  air,  Table  XXXIX.  The 
general  results  were  confirmed  by  G.  Meyer,2  Table  XL,  who  made 

TABLE  XXXIX. — SURFACE  TENSION  OF  MERCURY 
Data  in  dynes /cm  with  fresh  and  old  surfaces 


Medium 

Rapid 

After  60' 

Vacuum  (15°) 

436 

434 

Hydrogen  (21°)  

470 

436 

Oxygen  (25°) 

478 

432 

Nitrogen  (16°)  
Carbon  dioxide  (19°) 

489 

438 
429 

Dry  air  (17°)   

476 

429 

Moist  air  (17°)  

481 

TABLE  XL. — SURFACE  TENSION  OF  MERCURY 
Data  in  dynes /cm  with  fresh  and  old  surfaces 


Medium 

Vibrating  jet 

Curvature  of 
surface 

Hydrogen                    

554 

470 

Oxygen  

504 

478 

Carbon  dioxide                       

487 

480 

Nitrogen 

496 

489 

Wied.  Ann.,  66,  49  (1898). 
Ibid.,  66,523  (1898). 


134  APPLIED  COLLOID  CHEMISTRY 

use  of  Rayleigh's  jet  method  for  the  rapid  measurements  and 
consequently  obtained  higher  values  than  Stockle. 

It  may  be  easy  to  see  that  the  adsorption  of  a  gas  by  mercury 
should  lower  the  surface  tension,  but  it  is  not  clear  why  different 
gases  should  bring  the  surface  tension  down  practically  to  the 
same  value  as  that  found  when  there  is  supposed  to  be  no  gas 
present.  The  dynamic  methods  should  start  at  the  value  for 
mercury  in  a  vacuum  if  no  factor  has  been  overlooked  and  there 
is  no  apparent  reason  why  an  unadsorbed  gas  should  raise  the  sur- 
face tension  of  mercury  so  much. 

Lenard1  has  brought  up  a  point  which  may  have  a  bearing  on 
this.  If  we  have  a  partially  polymerized  liquid  such  as  water,  the 
modification  having  the  lower  surface  tension  will  concentrate 
in  the  surface.  If  we  form  a  new  surface  suddenly,  we  shall  get 
a  higher  concentration,  temporarily,  of  the  particular  modification 
which  has  the  higher  surface  tension.  If  the  equilibrium  is 
reached  relatively  slowly,  the  dynamic  method  will  give  a  higher 
value  for  the  surface  tension  than  the  static  methods.  If 
equilibrium  is  reached  instantaneously,  there  will  be  no  dif- 
ference. We  can  account  for  the  facts  observed  with  mercury  if 
we  make  the  assumptions  that  mercury  is  a  partially  polymer- 
ized liquid,  that  equilibrium  between  the  modifications  is  reached 
practically  instantaneously  in  a  vacuum,  and  that  equilibrium  is 
reached  relatively  slowly  in  presence  of  gases.  The  difficulty  with 
this  is  that  we  have  no  independent  proof  of  these  assumptions. 

Another  hypothetical  explanation  becomes  possible,  if  we 
assume  that  there  is  not  a  sharp  discontinuity  at  the  surface 
between  liquid  and  vapor.  The  Laplace  theory  of  surface  tension 
assumes  that  there  is  a  perfectly  sharp  line  of  demarcation  be- 
tween the  two  media  bounding  the  surface,  for  instance,2  between 
liquid  and  air,  while  van  der  Waals  postulates  a  continuous 
transition. 

According  to  the  latter  way  of  looking  at  things,  mercury  in  a 
vacuum  adsorbs  its  own  vapor  forming  a  thin  transition  layer 
varying  from  the  density  of  liquid  mercury  at  one  side  to  the 
density  of  mercury  vapor  at  the  other  side.  If  this  transition 

1  Cf.  AGANIN:  Drude's  Ann.,  45,  1020  (1914). 

2  WILLOWS  and  HATSCHBK:  Surface  Tension  and  Surface  Energy,    33 
(1915). 


SURFACE  TENSION— BROWNI AN  MOVEMENTS          135 

film  forms  instantaneously  in  a  vacuum  and  relatively  slowly  in 
presence  of  a  gas,  the  surface  tension  of  a  fresh  surface  of  mercury 
in  presence  of  a  gas  will  be  higher  than  the  equilibrium  surface 
tension  of  mercury  in  a  vacuum,  and  this  higher  surface  tension 
thus  measured  will  decrease,  if  the  surface  is  not  renewed,  down 
to  the  equilibrium  surface  tension  of  mercury  in  a  vacuum,  or 
below  it  if  the  gas  is  adsorbed  markedly  at  the  mercury  surface. 
This  explanation  seems  somewhat  more  plausible  than  that  of 
Lenard;  but  it  is  open  to  the  same  objection  that  there  is  as  yet 
no  independent  proof  of  the  assumptions  involved. 

Ferguson1  has  shown  that  the  surface  tension  of  benzene  is 
different  in  air  from  what  it  is  in  carbon  dioxide.  It  is  a  question 
of  course  to  what  extent  this  difference  is  due  to  the  solubilities 
of  the  gases  in  the  benzene.  Ferguson  considers  that  the  nature 
of  the  gas  is  more  important  than  the  solubility  effect;  but  he 
gives  no  evidence  for  this.  He  does,  however,  mention  one 
interesting  and  surprising  fact  that  the  absolute  value  of  the 
surface  tensions  increased  slightly  as  the  liquid  became  more  and 
more  saturated,  finally  reaching  a  steady  value. 

From  the  fact  that  the  lowering  of  the  surface  tension  is 
accompanied  by  an  increase  in  the  concentration  of  the  surface 
film  of  a  true  solution,  Freundlich2  draws  the  conclusion  that 
this  is  true  in  all  cases,  and  that  if  a  dissolved  substance  lowers 
the  surface  tension  between  the  solid  and  the  solution,  the  dis- 
solved substance  is  adsorbed;  conversely  that  the  dissolved 
substance  is  adsorbed  only  in  case  it  lowers  the  surface  tension 
at  the  interface.  The  conclusion  may  or  may  not  be  right;  but 
the  logic  is  faulty  because  the  two  cases  are  not  parallel.  The 
Gibbs  relation  holds  for  a  true  solution,  where  the  difference  of 
concentration  is  entirely  inside  the  solution  phase.  Freundlich  is 
extending  the  relation  to  a  heterogeneous  system  in  which  the 
adsorbed  substance  is  presumably  on  the  outer  surface  of  the 
solid  and  by  definition  cannot  diffuse  into  it  because  we  should 
then  have  a  solid  solution.  It  is  very  difficult  to  prove  or  dis- 
prove Freundlich's  conclusion  experimentally;  but  it  is  a  great 
pity  to  have  people  believe,  as  many  now  do,  that  the  generaliza- 
tion is  based  on  sound  thermodynamics. 

1  Phil.  Mag.  (6)  28,  403  (1914). 
2Zeit.  phys.  Chem.,   67,  424  (1907). 


136  APPLIED  COLLOID  CHEMISTRY 

The  surface  tension  of  solids  cannot  be  measured  directly;  but 
Tangl1  has  made  some  experiments  on  the  surface  tension  of 
such  pseudo-solids  as  rubber  and  paraffin  against  water. 

Solids  undoubtedly  do  have  definite  surface  tensions.  In 
fact,  one  might  consider  the  melting  point  as  the  temperature 
at  which  the  surface  tension  overcomes  the  crystalline  forces. 
In  the  liquid  crystals  and  the  flowing  crystals,  we  have  interme- 
diate stages.  Reference  has  already  been  made  to  Beilby's 
experiments  on  thin  gold  leaf  which  became  granular  when  heated.2 
We  shall  see  later  that  the  phenomenon  of  sintering  depends  on 
surface  tension. 

BROWNIAN  MOVEMENTS 

If  a  stone  be  dropped  into  water,  it  sinks  rapidly;  but  if  it  be 
ground  into  fine  particles,  the  surface  is  much  greater  and 
consequently  the  particles  sink  slowly.  If  the  stone  were  ground 
into  very  fine  particles,  we  should  expect  these  to  sink  very  slowly, 
the  rate  being  a  function  of  the  diameter  and  density  of  the 
particles.  A  formula, 

'          2r*(8-S')g 

&t 

for  this  case  has  been  deduced  by  Stokes,3  where  V  is  the  constant 
rate  of  fall,  r  the  radius  of  the  particles,  S  their  specific  gravity,  S' 
the  specific  gravity  of  the  liquid,  rj  its  viscosity  coefficient  and 
g  the  gravitational  constant.  Hatschek4  has  applied  this  formula 
to  several  special  cases.  For  particles5  of  gold  of  I/*  radius  the 
rate  of  fall  in  water  is  calculated  to  be  2.4  mm  per  minute.  With 
gold  particles  of  10/zju  radius  the  rate  of  settling  in  water  is  about 
10  mm  per  month.  For  a  particle  of  lOju/*  radius  and  a  specific 
gravity  of  only  3  the  calculated  rate  would  be  about  1  mm  per 
month.  Very  small  rates  of  fall  have  actually  been  observed. 

Crude's  Ann.,  34,  311  (1911);  cf.  JONES:  Zeit.  phys.  Chem.,  82,  448 
(1913);  RONTGEN:  Wied.  Ann.,  3,  321  (1878). 

2  Proc.  Roy.  Soc.,  72,  226  (1904). 

3  Mathematical  and  Physical  Papers,  3,  1  (1901).     For  modifications  and 
limitations  see  CUNNINGHAM:  Proc.  Roy.  Soc.,  83A,  357  (1910);  Jour.  Am. 
Chem.  Soc.  41,  319  (1919);  MILLIKAN:  Phys.  Zeit.,  11,  1097  (1910). 

*  An  Introduction  to  the  Physics  and  Chemistry  of  Colloids,  24  (1913). 
6  Elutriation,  or  the  difference  in  the  rate  of  settling,  is  much  used  in 
grading  powders  such  as  emery,  carborundum,  ultramarine,  kaolin,  etc. 


SURFACE  TENSION— BROWNI AN  MOVEMENTS         137 

Even  in  air  fine  particles  settle  very  slowly.  A  sandstorm  in 
the  Grand  Canyon  may  make  the  air  a  brick-red  color  for  two  or 
three  days.1  Similar  phenomena  are  to  be  observed  all  over  the 
world.2 

Ebell3  states  that  ultramarine  will  stay  suspended  in  pure 
water  for  months  when  obtained  in  a  very  finely  divided  state 
by  grinding  and  elutriation.  Mlilhauser4  found  that  when  very 
finely  ground  carborundum  powder  is  treated  with  water,  a  por- 
tion of  it  does  not  settle  in  months  and  behaves  in  that  respect 
like  colloidal  metallic  silver.  Brewer5  points  out  the  slow  rate 
at  which  clays  settle,  and  raises  the  question  whether  extremely 
finely  divided  particles  settle  at  all. 

If  the  very  finely  divided  particles  do  not  really  settle  at  all, 
there  must  be  some  factor  which  neutralizes  the  action  of  gravity 
more  or  less  completely.  The  experiments  of  an  English  botanist, 
Brown,6  give  us  a  clue  to  the  difficulty,  though  the  importance 
of  these  experiments  was  not  appreciated  until  very  muqh  later. 
Brown  was  studying  under  the  microscope  the  grains  of  pollen 
from  Clarckia  pulchella,  and  observed  that  some  of  them  moved 
about  in  the  liquid.  Further  experiments  showed  that  prac- 
tically all  finely  divided  solids  kept  in  continuous  motion.  Brown 
considered  that  he  was  watching  the  movements  of  active 
molecules. 

The  zig-zag  movements  of  suspended  particles  are  known 
as  Brownian  movements.  They  are  apparently  due  to  the  bom- 
bardment of  the  suspended  particles  by  the  molecules  of  the 
liquid.7  On  this  hypothesis  the  motion  should  be  more  rapid, 
the  smaller  the  bombarded  particles  and  this  proves  to  be  the 
case.  The  work  of  Gouy  showed  that  the  hypothesis  of  molecular 
agitation  gave  a  satisfactory  explanation  of  the  Brownian  move- 

1  ELLSWORTH    and    EMERY    KOLB  :  National    Geographical    Magazine, 
26,  134  (1914). 

2  See  EHRENBERG:  Die  Bodenkolloide,  209  (1915). 

3  Ber.  deutsch.  chem.  Ges.,  16,  2429  (1883). 

4  Zeit.  anorg.  Chem.,  6,  117  U894). 

6  Mem.  Nat.  Acad.  Sci.,  2,  165  (1884);  Am.  Jour.  Sci.  (3)  29,  1  (1885). 

6  Phil.  Mag.,  4,  161  (1828);  6,  161  (1829). 

'WIENER:  Pogg.  Ann.,  118,  79  (1863);  GOUY:  Jour,  de  Phys.  (2)  7, 
561  (1888);  Comptes  rendus,  109,  102  (1889);  Revue  g4n6rale  des  Sciences, 
1895,  1. 


138  APPLIED  COLLOID  CHEMISTRY 

ments  and  that  all  the  other  tentative  explanations  were  faulty. 
He  showed  that  the  Brownian  movements  are  not  due  to  vibra- 
tions transmitted  to  the  liquid  under  examination,  since  they 
persist  equally  as  well,  for  example,  at  night  in  the  country  as  during 
the  day  near  a  populous  street  where  heavy  vehicles  pass.  They  are 
not  due  to  convection  currents  existing  in  fluids  where  thermal 
equilibrium  has  not  been  attained,  for  they  do  not  change  ap- 
preciably when  plenty  of  time  is  given  for  equilibrium  to  be 
reached.  Any  comparison  between  Brownian  movements  and 
the  agitation  of  dust  particles  dancing  in  the  sunlight  must 
therefore  be  set  aside.  In  addition,  in  the  latter  case,  it  is  easy 
to  see  that  the  neighboring  dust-particles  move  in  general  in 
the  same  sense,  roughly  tracing  out  the  form  of  the  common 
current  which  bears  them  along,  whereas  the  most  striking  feature 
of  the  Brownian  movements  is  the  absolute  independence  of 
the  displacement  of  neighboring  particles,  so  near  together  that 
they  pass  by  one  another.  The  movements  cannot  be  due  to  an 
effect  of  light  because  Gouy  was  able  to  vary  the  intensity  of  the 
illumination  abruptly  a  thousand-fold,  or  to  change  the  color 
considerably  without  modifying  the  phenomena.  The  nature  of 
the  particles  does  not  appear  to  be  of  any  importance,  and  it  is 
therefore  difficult  not  to  believe  that  these  particles  reveal 
an  internal  agitation  of  the  fluid,  the  better  the  smaller  they  are, 
much  as  a  cork  follows  better  than  a  large  ship  the  movements  of 
the  waves  of  the  sea. 

The  distance  between  the  two  end-points  as  a  particle  zig- 
zags forward  and  back  is  usually  defined  as  twice  the  amplitude. 
Svedberg1  found  that  for  different  solutions  the  product  of  the 
amplitude,  A,  into  the  viscosity,  17,  is  a  constant  and  that  the 
amplitude  divided  by  the  time  which  the  particles  take  in  passing 
through  that  distance  is  also  a  constant.  These  results  are  in 
qualitative  agreement  with  a  theoretical  formula  deduced  by 
Einstein.2  If  one  knows  the  amplitude  and  the  time,  one  can 
calculate  the  speed  of  the  particles.  For  platinum  particles  with 
a  diameter  of  10-50/i/*,  Svedberg  found  speeds  of  200-400^ 
per  second.  Perrin3  considers  that  errors  of  measurements  vi- 

1  Zeit.  Elektrochemie,  12,  835,  909  (1906). 

2  Drude's  Ann.,  17,  549  (1905);  19,  289,  371  (1906). 

3  Brownian  Movement  and  Molecular  Reality,  22  (1910). 


SURFACE  TENSION— BROWNIAN  MOVEMENTS         139 

tiate  these  data.  He  says  that  the  speed  of  a  granule  in  suspen- 
sion cannot  be  measured  accurately. 

Even  though  quantitative  measurements  may  be  inaccurate, 
it  is  desirable  to  have  a  general  idea  of  the  magnitude  of  the  phe- 
nomenon. From  Exner's1  experiments  it  appears  that  particles 
with  a  diameter  larger  than  4/*  show  no  perceptible  Brownian 
movements.  Particles  smaller  than  O.lju  begin  to  show  lively 
movements,2  while  particles  with  diameters  of  about  lO^Ai  give 
apparent  trajectories  up  to  20^. 

Perrin3  came  to  the  conclusion  that  the  gas  laws  must  apply  to 
dilute  emulsions.  "Let  us  suppose  that  it  is  possible  to  obtain 
an  emulsion,  with  the  granules  all  identical,  an  emulsion  which 
I  shall  call  for  shortness,  uniform.  It  appeared  to  me,  at  first 
intuitively,  that  the  granules  of  such  an  emulsion  should  dis- 
tribute themselves  as  a  function  of  the  height  in  the  same  manner 
as  the  molecules  of  a  gas  under  the  influence  of  gravity.  Just 
as  the  air  is  more  dense  at  sea-level  than  on  a  mountain-top,  so 
the  granules  of  an  emulsion,  whatever  may  be  their  initial  dis- 
tribution, will  attain  a  permanent  state  where  the  concentration 
will  go  on  diminishing  as  a  function  of  the  height  from  the  lower 
layers;  and  the  law  of  rarefaction  will  be  the  same  as  for  air." 

To  test  this  hypothesis  Perrin  prepared  a  uniform  suspension 
of  purified  gamboge  in  water  by  means  of  fractional  centrifug- 
ing.  The  results  confirmed  the  hypothesis.  With  gamboge 
particles  0.3ju  in  diameter  a  rise  of  30/*  was  sufficient  to  lower  the 
concentration  to  a  half  of  its  value,  30/x  in  the  gamboge  suspen- 
sion being  equivalent  to  6  kilometers  in  the  air.4  Satisfactory- 
results  have  also  been  obtained  by  Zangger5  for  drops  of  mercury, 
by  Brillouin6  for  gamboge  in  glycerol  solutions  having  a  viscosity 
160  times  that  of  water,  and  by  Iljin.7  This  cannot  be  true  over 
any  appreciable  range  because  it  would  mean  that  the  concentra- 
tion would  drop  to  one-billionth  in  a  rise  of  about  a  millimeter, 
which  would  mean  that  no  colloidal  solution  would  appear  uni- 

1  Sitzungsber.  Akad.  Wiss.  Wien.,  56  II,  116  (1867). 

2  ZSIGMONDY:  Kolloidchemie,  18  (1912). 

3  Brownian  Movement  and  Molecular  Reality,  22  (1910). 

4  PERRIN  :  Brownian    Movement    and    Molecular    Reality,    43     (1910). 
6Zeit.  Kolloidchemie,  7,  216  (1911). 

6  Ann.  Chim.  Phys.  (8)  27,  412  (1912). 

7  Zeit.  phys.  Chem.,  83,  592  (1913). 


140  APPLIED  COLLOID  CHEMISTRY 

form.  It  has  been  suggested  by  Burton  that  the  concentration 
increases  with  increasing  depth  only  up  to  about  a  certain  value 
and  then  remains  practically  constant. 

Einstein1  assumes  that  suspended  particles  will  behave  exactly 
like  dissolved  particles,  exerting  an  osmotic  pressure  and  diffusing 
through  the  solvent.2  Instead  of  the  van't  Hoff  expression  that 
the  osmotic  pressure,  P,  of  a  dissolved  substance  is  given  by  the 
equation 

PV  =  nRT 

Einstein  deduces  the  equation 

PV  —  —  ET 

N 

where  n  is  now  the  number  of  suspended  particles  and  N  is 
Avogadro's  constant  or  the  actual  number  of  molecules  in  a 
gram-molecule.  Following  this  up  he  deduces  for  the  rate  of 
diffusion  the  equation 

r>  /RT   t_ 

:  \  NT  Sri, 

where  D*  is  the  displacement  in  a  given  direction,  t  is  the  time, 
R  is  the  gas  constant,  T  the  absolute  temperature,  N  is  the  Avo- 
gadro  constant,  r  is  the  radius  of  the  particles  a*id  77  is  the  vis- 
cosity of  the  liquid.  This  has  been  confirmed  quantitatively  by 
Perrin  who  gets  a  mean  value3  of  about  68  X  1022  for  N.  In 
regard  to  this  he  says:4  "Thus  the  molecular  theory  of  the 
Brownian  movement  can  be  regarded  as  established  experi- 
mentally, and  it  becomes  very  difficult  to  deny  the  objective 
reality  of  molecules.  At  the  same  time  we  see  the  law  of  gases, 
already  applied  by  van't  Hoff  to  dilute  solutions,  extended  to 
uniform  emulsions.  The  Brownian  movement  offers  us,  on  a 
different  scale,  the  faithful  picture  of  the  movements  possessed, 

1  Drude's  Ann.,  17,  549  (1905);  19,  280,  371  (1906).     See  also  SMOLU- 
CHOWSKI:  Ibid.,  21,  756  (1906). 

2  It  must  be  remembered  that  Einstein  has  ignored  all  adsorption  of  the 
liquid  by  the  suspended  particles.     He  did  the  same  thing  when  deducing 
a  formula  for  the  viscosity  of  colloidal  solutions  and  came  to  the  erroneous 
conclusion  that  the  viscosity  depends  only  on  the  volume  and  not  on  the  size 
of  the  suspended  particles. 

3  PERRIN:  Die  Atome,  123  (1914). 

4  PERRIN:  Brownian  Movement  and  Molecular  Reality,  46  (1909). 


SURFACE  TENSION— BROWNTAN  MOVEMENTS  141 

for  example,  by  the  molecules  of  oxygen  dissolved  in  the  water  of 
a  lake,  which,  encountering  one  another  only  rarely,  change  their 
direction  and  speed  by  virtue  of  their  impacts  with  the  molecules 
of  the  solvent. 

"It  may  be  interesting  to  observe  that  the  largest  of  the 
granules,  for  which  I  have  found  the  laws  of  perfect  gases  fol- 
lowed, are  already  visible  in  sunlight  under  a  strong  lens.  They 
behave  as  the  molecules  of  a  perfect  gas,  of  which  the  gram- 
molecule  would  weigh  200,000  tons. 

"I  add  lastly  that  all  the  measurements  detailed  in  this  para- 
graph have  been  made  on  dilute  emulsions,  which  in  the  parts 
richest  in  granules  only  contain  a  thousandth  part  of  resin,  and 
where  the  osmotic  pressure  does  not  reach  a  thousand  millionth 
of  an  atmosphere." 

If  we  substitute  four  times  the  amplitude  for  Dx(Da)=  4A)  in 
Einstein's  formula,  t  will  be  the  time  required  for  the  particle  to 
make  one  complete  oscillation.1  Einstein's  formula  there- 
fore becomes 

...^        TrRT    t 
(4A)         -N"3Si 

If  we  are  considering  particles  of  the  same  size  in  various  liquids 
we  may  write  the  equation  A2  =  k.t/rj  if  we  are  working  at 
constant  temperature.  "  Since,  by  the  kinetic  theory,  the 
average  kinetic  energy  of  a  particle  is  the  same  as  the  average 
kinetic  energy  of  a  molecule,  and  the  molecules  of  all  fluids  at 
the  same  temperature  have  the  same  average  kinetic  energy,  we 
should  expect  the  average  velocity  of  all  the  particles  to  be  the 
same,  provided  their  masses  are  the  same,  whatever  the  liquid 
in  which  they  are  moving."  Einstein's  formula  therefore  leads 
to  the  set  of  relations:  Kr\  =  const.;  Dx  =  \/T/5  and  D«/\/t' 
=  const. 

As  has  been  pointed  out,  Svedberg  had  discovered  independ- 
ently that  the  product  of  the  amplitude  into  the  viscosity  was  a 
constant  at  constant  temperature.  Exner2  found  that  tl\e  ratio 
of  displacements  at  20°  and  71°  is  1:1.6  while  the  ratio  of  the 
corresponding  values  of  x/T/r?  for  water  at  these  two  tempera- 

1  BURTON:  The  Physical  Properties  of  Colloidal  Solutions,  76  (1916). 
2 BURTON:  The  Physical  Properties  of  Colloidal  Solutions,  77  (1916). 


142  APPLIED  COLLOID  CHEMISTRY 

tures  is  1:1.7.  Seddig1  has  shown  by  means  of  a  photographic 
method  in  conjunction  with  the  microscope,  that  finely  divided 
cinnabar  in  water  obeys  Einstein's  formula,  the  greatest  depar- 
ture from  the  theoretical  value  being  6  percent. 

Chaudesaigues,2  working  with  an  emulsion  of  gamboge,  varied 
the  viscosity  of  a  solution  by  adding  sugar.  In  this  way  he  carried 
out  observations  on  two  solutions  containing  the  same-sized 
particles  at  the  same  temperature,  the  viscosities  of  which  were 
in  the  ratio  1 : 4  and  found  that  the  motion  was  twice  as  fast  in  the 
less  viscous  solution  as  it  was  in  the  solution  of  higher  viscosity. 
Chaudesaigues  observed  the  velocities  of  particles  of  gamboge 
with  radii  respectively  4.5  X  10~5  cm  and  2.3  X  10~5,  practically 
a  ratio  of  2:1,  and  found  that  these  velocities  varied  inversely  as 
the  square  of  the  radii.  He  also  observed  the  distances  described 
by  50  grains,  each  of  radius  2.13  X  10~5  cm  during  successive 
intervals  of  30  seconds  each  and  obtained  the  following  results: 
on  the  average  the  particles  moved 

6.7,  9.3,  11.8  and  13.95  microns  in 

30,  60,  90  and  120  seconds.     The  square  roots  of  the 
times  are  proportional  to  the  numbers, 

6.7,  9.46,  11.6  and  13.4,  giving  an  ideal  confirmation  of 
this  point. 

It  was  first  pointed  out3  by  Smoluchowski4  that  we  should 
expect  to  have  the  Brownian  movements  in  gases  as  well  as  in 
liquids  and  he  quotes  from  Bodaszewski5  and  0.  Lehmann6  as  to 
the  dancing  movements  executed  by  the  particles  of  fumes  of 
ammonium  chloride,  acids,  phosphorus,  etc.  which  they  com- 
pared to  the  Brownian  movements  in  liquids  and  interpreted  as 
molecular  movements.7  The  formulas  of  Einstein  and  Smolu- 
chowski are  based  on  the  Stokes  law,  which  does  not  hold  for  gases 
under  low  pressure  where  the  mean  free  path  of  the  gas  molecules 


1  Phys.  Zeit.,  9,  465  (1908). 

2  Comptes  rendus,  147,  1044  (1908). 

3  BURTON:  The  Physical  Properties  of  Colloidal  Solutions,  85  (1916). 

4  Drude's  Ann.,  21,  756  (1906). 

6  Kosmos,  7,  177  (1882);  Beibl.,  8,  488  (1883). 

6  Molekularphysik,  II,  5. 

7  "  The  sun,"  says  Daniel  Culverwell,  "discovers  atoms,  though  they  be 
invisible  by  candle  light,  and  makes  them  dance  naked  in  his  beams." 


SURFACE  TENSION— BROWNIAN  MOVEMENTS 


143 


is  large  relatively  to  the  radius  of  the  particles.  Ehrenhaft1  was 
the  first  to  carry  out  direct  measurements  on  the  Brownian 
movement  of  particles  in  gases.  He  found  that,  as  theory  pre- 
dicts, there  is  a  much  livelier  motion  in  gases  than  in  liquids;  at 
the  same  time  the  action  of  gravity  in  causing  vertical  descent 
of  the  particles  is  also  much  more  apparent  in  gases  than  it  is  in 
liquids.  For  comparatively  large  particles  in  the  smoke  of  cigars 
and  cigarettes  and  in  the  fumes  of  ammonium  chloride,  he  ob- 
served an  undoubtedly  zigzag  Brownian  movement,  while  with 
smaller  particles  obtained  by  a  spark  discharge  between  zinc, 
platinum,  or  silver  electrodes,  Ehrenhaft  concluded  that  he  has 
proved  without  a  doubt  the  existence  of  a  motion  in  gases 
completely  analogous  to  the  Brownian  movement  in  liquids. 

It  is  more  difficult  to  recognize  this  motion  in  gases  on  account 
of  disturbing  convection  currents  and  the  action  of  gravity. 
For  the  heavier  particles,  the  velocity  due  to  the  force  of  gravity 
completely  overshadows  that  due  to  the  molecular  shocks,  while 
the  opposite  is  the  case  with  the  very  small  particles.  In  Table 
XLI  are  given  the  velocities  impressed  by  molecular  shocks  and 
by  the  force  of  gravitation,  respectively,  on  silver  particles  of 

TABLE  XLI. — BROWNIAN  MOVEMENTS  IN  GASES 


Radius  of  silver 
particles 

Velocity  due  to 
molecular  shocks 

Velocity  due  to 
gravity 

IMM 

63  .  OM/SCC. 

1  .  2ju/t/sec. 

SMM 

28  .  (V/sec. 

30.  (W  /sec. 

1(W 

20.0M/sec. 

120  .  0/z/i/sec. 

5(W 

8  .  9M/sec. 

3.0M/sec. 

10(W 

6.3M/sec. 

12.0M/sec. 

50(W 

2  .  8/i/sec. 

300.  0M  /sec. 

In 

2.(V/sec. 

1200.0M/sec. 

various  sizes  (density  10.5) .  These  numbers  show  that  when  we 
reach  particles  having  diameters  of  the  order  of  the  wave-length 
of  light  the  two  velocities  do  not  differ  materially.  As  we  deal 
with  smaller  particles,  the  velocity  induced  by  gravity  soon 
becomes  negligible,  while  with  particles  increasing  above  O.lju, 
the  motion  due  to  gravity  soon  dominates  the  situation.  This  is 
1  Sitzungsber.  Akad.  Wiss.  Wien.,  116  Ho,  1139  (1907). 


144  APPLIED  COLLOID  CHEMISTRY 

in  keeping  with  what  Ehrenhaft  found.  Particles,  the  linear 
dimensions  of  which  were  of  the  order  of  the  size  of  the  mean  free 
path  of  gas  molecules  (0.1/x),  and  somewhat  larger  particles,  fall 
in  a  zig-zag  line,  the  velocity  due  to  gravitation  being  greater 
than  that  due  to  molecular  shocks.  Particles,  which  are  near  the 
limit  of  visibility  in  the  ultra-microscope  (IMM),  are  in  such  lively 
molecular  motion  that  the  vertical  gravitational  velocity  is 
completely  masked.  Ehrenhaft  viewed  silver  particles  that 
remained  in  lively  motion  in  the  air  of  his  ultra-microscope  cell 
for  some  thirty  minutes.  His  measurements  on  the  cigarette 
smoke  particles  gave  a  mean  velocity  of  25/*/sec.  and  on  the 
smaller  particles  of  silver,  46/-i/sec. 

Much  interesting  work  has  been  done  in  this  field  by  De 
Broglie.1  Both  Ehrenhaft  and  De  Broglie  have  found  that  these 
silver  particles  suspended  in  air,  are  charged  and,  by  measuring 
the  velocity  impressed  on  them  by  a  known  electric  field,  have 
come  to  the  conclusion  that  the  charge  is  that  of  one  electron. 
The  values  that  they  find  for  this  charge,2  e,  are  respectively  4.6 
X  10-10  and  4.5  X  lO"10  electrostatic  units.  Wells  and  Gerke3 
have  measured  the  velocity  of  smoke  particles  in  an  electrical 
field  and  have  determined  the  size  of  the  particles  from  the 
resulting  data. 

It  seems  to  be  accepted  pretty  generally  that  the  work  of 
Perrin,  Svedberg,  and  others  has  established  the  practical  con- 
tinuity between  suspended  particles  and  dissolved  substances; 
but  this  seems  to  be  an  overhasty  conclusion.  The  molecular 
weight  of  a  dispersed  substance,  as  calculated  from  any  of  Ein- 
stein's formulas  means  something  entirely  different,  if  it  means 
anything  at  all.  We  can  determine  the  molecular  weight 
of  liquid  benzene  approximately  from  a  measurement  of  the 
surface  tension;  but  it  is  absurd  to  say  that  suspending  fine  drops 
of  benzene  in  water  causes  the  molecular  weight  of  benzene  to  be 
equal  to  200,000  tons.4  We  are  talking  about  entirely  different 
things  in  the  two  cases.  What  we  mean  is  that  dissolved  ben- 

1  Comptes  rendus,  146,  624,  1010  (1908);  148,  1163,  1315  (1909). 

2  See  also  MILLIKAN:  Phil.  Mag.  (6)  19,  209  (1910). 

3  Jour.  Am.  Chem.  Soc.,  41,  312  (1919). 

4  Such  a  hypothetical  gas  would  have  a  density  roughly  a  million  times 
that  of  metallic  copper. 


SURFACE  TENSION— BROWNIAN  MOVEMENTS         145 

zene  has  a  molecular  weight  of  78,  and  that  liquid  benzene  sus- 
pended in  water  behaves  or  may  behave  as  if  it  were  a  dissolved 
substance  having  a  molecular  weight  of  200,000  tons,  more  or  less. 
As  a  matter  of  fact  the  experiments  bring  out  clearly  the  enor- 
mous difference  between  a  solution  and  a  suspension. 

The  safe  ground  to  take  is :  that  the  Brownian  movements  are 
due  to  the  incessant  movements  of  the  molecules  of  the  fluid; 
that  the  Brownian  movements  tend  to  make  finely-divided, 
suspended  particles  distribute  themselves  uniformly  throughout 
the  liquid;  that  the  uniform  distribution  is  affected  by  the  force 
of  gravity  as  in  the  case  of  a  gas ;  and  that  the  Brownian  move- 
ments, though  causing  diffusion,  give  rise  to  no  appreciable 
osmotic  pressure. 

Very  finely  divided  particles  (less  than  0.5/*  for  instance)  will 
be  kept  in  suspension  indefinitely  by  the  Brownian  movements, 
so  long  as  the  particles  remain  finely  divided.  If,  however,  two 
or  more  particles  agglomerate  or  coalesce,  the  force  of  gravity 
may  cause  the  particles  to  settle  to  the  bottom  of  the  containing 
vessel. 


10 


CHAPTER  V 
COALESCENCE 

COALESCENCE  OF  LIQUIDS 

If  two  small  drops  of  a  pure  liquid  are  brought  in  contact, 
they  run  together  and  form  a  larger  drop.  The  two  small  drops 
are  therefore  instable  with  respect  to  a  larger  drop.  Conse- 
quently we  should  expect  the  small  drop  to  distill  over  to,  and 
condense  on,  a  larger  drop.  This  is  equivalent  to  saying  that  the 
greater  the  curvature  of  the  drop,  the  higher  the  vapor-pressure. 
This  conclusion  was  drawn  by  Lord  Kelvin,  then  William  Thom- 
son,1 many  years  ago.  He  reached  his  conclusion  in  a  slightly 
different  way.  If  we  have  a  capillary  tube  dipping  into  a  liquid, 
the  whole  being  in  a  closed  vessel  containing  only  the  liquid  and 
its  vapor,  the  raised  liquid  surface  in  the  capillary  must  be  in 
equilibrium  with  the  main  surface  of  the  liquid  if  the  system  is  all 
at  the  same  temperature.  The  pressure  of  the  vapor  varies, 
however,  at  different  levels  in  consequence  of  gravity.  Con- 
sequently the  concave  surface  of  the  liquid  meniscus  must  have 
a  lower  vapor-pressure  than  the  plane  surface  of  the  liquid.  The 
converse  must  also  be  true  that  the  convex  surface  of  a  drop 
must  have  a  higher  vapor-pressure  than  the  plane  surface  of  the 
liquid.  This  difference  will  be  greater  the  greater  the  curvature 
of  the  surface  or,  in  other  words,  the  smaller  the  drop.  The 
actual  difference  in  vapor  pressure  between  the  highly  curved 
surface  in  a  capillary  tube  and  the  nearly  plane  surface  in  a  wide 
vessel  is  very  small.  It  amounts  to  one-thousandth  of  the 
vapor-pressure  in  a  capillary  tube  in  which  water  would  rise 
thirteen  meters. 

Lord  Kelvin2  considered  that  this  theory,  originally  due  to 
Laplace,  is  applicable  to  cases  in  which  the  radius  of  curvature  is 


1  Phil.  Mag.  (4)  42,  448  (1871). 

2  Ibid.  (4)42,451  (1871). 


146 


COALESCENCE  147 

as  small  as  1.2ju;  but  he  thinks  that  we  are  not  entitled  to  push 
it  much  further.  Consequently  he  believes  that  the  formulas  he 
has  deduced  are  not  applicable  to  the  vapor  pressures  of  moisture 
retained  by  such  substances  as  cotton  cloth  and  oatmeal  at 
temperatures  far  above  the  dewpoint  of  the  surrounding  atmos- 
phere. He  believes,  however,  that  the  difference  is  quantitative 
and  not  qualitative;  that  the  adsorption  of  vapor  by  fibrous  and 
cellular  organic  structures  is  a  property  of  matter  continuous 
with  the  adsorption  of  vapor  into  a  capillary  tube. 

The  converse  of  the  theory  is  also  true  that  liquid  will  distill 
into  a  capillary  tube  closed  at  the  lower  end  until  the  height  of 
the  meniscus  above  the  level  of  the  liquid  is  the  same  as  the 
height  to  which  the  liquid  would  have  risen  in  the  capillary  tube 
if  the  lower  end  had  been  open.  Bacon1  has  done  the  experiment 
successfully  with  ether,  though  the  rate  of  distillation  is  very  low. 

The  rate  at  which  small  drops  coalesce  depends  on  the  mobility 
of  the  liquid.  With  viscous  liquids  such  as  cooling  lavas  or 
slags,  we  know  that  we  get  all  sorts  of  phenomena  which  do  not 
correspond  to  equilibrium  relations,  and  it  is  probable  that 
jellies  are  rarely  in  true  equilibrium. 

COALESCENCE  OF  SOLIDS 

While  it  is  perfectly  familiar  to  all  of  us  that  two  small  drops 
of  a  pure  liquid  will  coalesce  readily  to  form  a  larger  drop,  all  our 
ordinary  experience  is  the  other  way  in  regard  to  solids  and  yet, 
theoretically,  finely  divided  solids  behave  like  finely  divided 
liquids  in  many  respects.  A  very  finely  divided  powder  must 
have  a  higher  vapor-pressure,  a  lower  melting-point,  and  a  greater 
solubility2  than  the  same  substance  in  a  more  coarsely  crystalline 
form.  When  yellow  phosphorus  is  placed  in  a  tube,  the  air 
removed,  and  the  tube  then  sealed,  it  is  said  by  Herman2  and  by 
J.  Lawrence  Smith3  that  crystals  form  in  a  short  time  on  the 
walls  of  the  tube  and  may  grow  to  a  diameter  of  3-5  mm 
in  the  course  of  a  month  or  six  weeks.  Crystals  of  iodine  having 
a  diameter  of  2-3  mm  were  kept  in  the  dark  for  eight  years, 

1  Phys.  Rev.,  20,  1  (1905). 

2OsTWALD:  Zeit.  phys.  Chem.,  36,  495  (1900). 

3  Jour.  Chem.,  Soc.,  27,  869  (1874). 


148  APPLIED  COLLOID  CHEMISTRY 

the  temperature  varying  from  0°  to  24°.  At  the  end  of  that  time 
von  Jonstorff1  found  that  the  smaller  crystals  had  mostly  disap- 
peared and  that  the  crystals  were  now  about  4-5  mm  in  diameter. 
Hautef  euille  and  Perrey2  found  that  at  a  dull  red  heat  hydrochloric 
acid  gas  at  a  pressure  of  three  atmospheres  causes  amorphous 
alumina  to  change  into  corundum,  titania  into  anatase  and  zir- 
conia  into  a  crystalline  form.  The  crystallization  of  the  alumina 
is  believed  to  be  due  to  the  successive  formations  and  decomposi- 
tions of  an  aluminum  chloride.  Emden3  has  shown  that  ice 
grains  as  large  as  hazel  nuts  may  be  grown  from  snow  slush  with 
original  small  ice  nuclei,  kept  in  sealed  vessels  for  several  weeks 
at  a  constant  temperature  of  0°. 

Pawlow4  states  that  crystals  of  salol  which  were  2,u  in  diameter 
melted  1.1°  lower  than  crystals  40/4  in  diameter.  Comparing  a 
fine  dust  of  less  than  2/u  diameter  with  crystals  0.5-2.0  mm 
diameter,  differences  of  4°-7°  were  obtained  with  salol,  antipyrin, 
and  phenacetin;  but  it  seems  probable  that  part  of  these  differ- 
ences is  due  to  impurities  and  part  to  experimental  error. 

Hulett5  found  practically  no  difference  for  the  solubility  of 
calcium  sulphate  in  water  until  the  radius  of  the  crystals  was  less 
than  2/4.  Changing  from  2/4  to  0.3/4  caused  an  increase  in  solu- 
bility of  nearly  20  percent.  A  coarsely  crystalline  barium 
sulphate  (1.8/4)  was  soluble  2.29  mg  per  liter  at  25°.  When 
ground  in  an  agate  mortar  to  O.l/i  the  solubility  rose  to  4.5  mg 
per  liter.  The  corresponding  figures  with  natural  barytes  were 
2.38  mg  and  6.18  mg.  Red  mercuric  oxide  has  a  solubility  of  50 
mg  per  liter  at  25°;  when  ground  to  a  fine  powder  the  color 
becomes  yellow  and  the  solubility  increases  to  150  mg  per  liter. 
Jones  and  Partington6  made  conductivity  measurements  on 
saturated  gypsum  solutions.  They  found  no  variation  with  size 
of  crystals  when  the  radius  exceeded  50/i. 

This  parallelism  between  small  grains  and  small  drops  makes 
one  wonder  whether  the  apparent  failure  of  solid  particles  to 

1  Ber.  deutsch.  chem.  Ges.,  10,  866  (1877). 

2  Comptes  rendus,  110,  1028  (1890). 

3  VON  ENGELN:  Am.  Jour.  Sci.  (4)  40,  461  (1915). 
4Zeit.  phys.  Chem.,  65,  1  (1908);  68,  366  (1909). 
6  Ibid.,  37,385  (1901). 

6  Jour.  Chem.  Soc.,  107  II,  1019  (1915). 


COALESCENCE  149 

coalesce  may  not  be  due  to  experimental  error.  If  we  precipitate 
copper  electrolytically  on  copper,  we  get  a  mass  which  does  not 
differ  materially  in  properties  from  cast  copper  having  the  same 
size  of  crystals.1  The  successive  layers  of  copper  coalesce  to  a 
uniform  mass.  On  the  other  hand  if  we  electroplate  nickel 
on  nickel  without  taking  special  precautions,  we  get  a  film  of 
nickel  which  does  not  adhere.  The  reason  for  this  is  that  the 
original  nickel  had  an  oxide  film  over  the  surface  and  the  electro- 
lytically precipitated  nickel  never  really  came  in  contact  with  the 
nickel  electrode.  If  the  oxide  film  is  removed,  the  plating  of 
nickel  on  nickel2  is  as  simple  as  the  plating  of  copper  on  copper. 
A  silver  halide  adheres  well  to  a  silver  anode  if  the  conditions 
are  such  that  the  halide  is  formed  in  actual  contact  with  the 
metal.3 

If  a  plate  were  broken  without  chipping,  the  two  pieces  would 
fit  together  perfectly  and  should  cohere,  showing  no  signs  of  a 
break.  This  does  not  happen  because  of  the  film  of  condensed 
air  which  acts  as  an  elastic  cushion  and  keeps  the  two  parts 
from  actually  being  brought  in  contact.4  A  corollary  from  this  is 
that  if  one  were  to  break  a  plate  in  an  absolute  vacuum  and  were 
to  bring  the  pieces  together  in  the  same  vacuum,  the  plate  should 
theoretically  be  as  good  as  new.  This  is  not  likely  to  happen 
practically,  because  we  cannot  get  an  absolute  vacuum.  If  we 
heat  the  fragments  of  any  broken  object,  several  things  happen. 
At  the  higher  temperature  less  gas  is  adsorbed  and  it  is  held  less 
firmly.  The  solid  is  likely  to  become  more  malleable  and  it  is 
therefore  easier  to  make  contact  at  a  number  of  points.  All 
chemists  know  that  it  is  easy  to  make  two  glass  rods  unite  at 
temperatures  at  which  the  glass  is  still  very  viscous.  It  may  be 
urged  that  glass,  at  those  temperatures,  is  unquestionably  a 
liquid  and  not  a  solid.  This  criticism  does  not  apply  to  plati- 
num. To  weld  platinum  it  is  only  necessary  to  put  two  clean 
pieces  of  platinum  in  contact,  to  heat  them  to  a  moderate  tem- 
perature far  below  the  melting-point  of  platinum,  and  to  give 
them  a  light  tap  with  a  hammer.  Since  the  object  in  these  cases 

1  BENNETT:  Jour.  Phys.  Chem.,  16,  294  (1912). 

2  SNOWDON:  Jour.  Phys.  Chem.,  10,  134  (1906). 

3  REEDY:  Am.  Jour.  Sci.  (4)  40,  407  (1915). 

«  See  BREUER:  Kitte  and  Klebstoffe,  23  (1907). 


150  APPLIED  COLLOID  CHEMISTRY 

is  to  bring  two  like  surfaces  into  intimate  contact,  it  is  easy  to 
see  why  clean  surfaces  are  essential  when  soldering  or  brazing 
metals,  and  why  it  is  important  to  use  a  flux.  When  iron  is 
treated  with  sodium  amalgam  as  a  preliminary  to  dry-gilding,1 
part  of  the  effect  of  the  sodium  amalgam  is  to  clean  the  surface 
of  the  iron.  Cold  sodium  cuts  readily  while  warm  sodium 
sticks  to  the  knife.2 

In  some  cases  two  surfaces  will  unite  fairly  readily.  This  is 
very  noticeable  with  unvulcanized  rubber  from  which  it  should 
follow  that  vulcanized  rubber  adsorbs  air  much  more  strongly. 
I  do  not  know  of  any  experiments  along  this  line.  Willows  and 
Hatschek3  point  out  that  copper  can  be  polished  to  such  a  degree 
that  a  cube  of  this  metal  will  support  eleven  others  merely  placed 
in  contact.  This  means  that  1  cm2  of  surface  carries  a  mass  of 
copper  11  cm  long  and  1  cm  square,  a  weight  of  about  98  grams. 
Polished  plane  surfaces  of  glass  are  apt  to  stick  together.  Usu- 
ally this  is  to  be  avoided;  but  Parker  and  Dalladay4  have  taken 
advantage  of  this  property.  Glass  surfaces  can  be  polished 
without  difficulty  so  as  to  be  so  nearly  plane  or  so  nearly  of  the 
same  curvature  that  they  show  less  than  one  interference  fringe 
per  inch  when  placed  together  and  examined  by  light  reflected 
from  the  interfaces.  These  surfaces  may  be  brought  into  optical 
contact  so  that  the  two  pieces  of  glass  act  optically  as  if  they  were 
one  block  by  applying  a  pressure  of  several  pounds  per  square 
inch.  The  two  pieces  can,  however,  be  separated  easily  by 
sudden  heating  or  cooling,  by  a  shearing  force,  or  by  the  applica- 
tion of  liquids  which  creep  in  between  the  surfaces.  By  selection 
of  the  proper  temperatures  and  pressures  it  has  been  found  pos- 
sible to  weld  two  such  pieces.  It  was  found  essential  that  the 
temperature  at  which  union  took  place  should  not  only  be  far 
below  the  melting-point  of  the  glasses  to  be  joined,  but  also 
below  the  annealing  temperature,  for  even  at  that  temperature 
a  very  slight  strain  would  cause  deformation  of  worked  glass 
surfaces.  At  60°  below  the  annealing  temperature  it  was  found 
that  two  pieces  of  glass  in  optical  contact  would  unite  within  an 

1  KIRCHMAN:  Jour.  Chem.  Soc.,  26,  418  (1873). 

2  MCDONALD:  Chem.  News,  97,  73  (1908). 

3  Surface  Tension  and  Surface  Energy,  8  (1915). 

4  Engineering,  103,  23;  Jour.  Franklin  Inst.,  183,  369  (1917). 


COALESCENCE  151 

hour  and  that  any  slight  deformation  produced  by  the  compres- 
sion could  be  remedied  by  a  few  strokes  of  the  polishing  tool.  . 

Mr.  Jerome  Alexander  has  called  my  attention  to  the  fact  that 
freshly  split  pieces  of  mica  can  be  made  to  coalesce  by  pressing 
them  between  the  fingers.  After  they  have  been  exposed  to  the 
air  for  five  or  ten  minutes,  it  is  impossible  to  make  them  unite  in 
this  way.  This  is  undoubtedly  due  to  the  formation  of  a  surface 
film,  because  Lord  Rayleigh1  found  that  freshly  split  surfaces  of 
mica  attract  to  themselves  at  once  a  coating  of  moisture  which 
is  replaced  or  supplemented  in  a  few  hours  by  a  layer  of  grease. 
The  fresh  surface  conducts  electricity,  while  an  old  surface,  in 
which  the  moisture  has  presumably  been  replaced  by  grease, 
insulates  well.  I  have  been  told  that  some  of  the  electrical 
companies  make  mica  blocks  by  compressing  scrap  mica;  but  I 
do  not  know  any  of  the  details  of  the  process.  Horn  waste  and 
tortoise-shell  waste  may  be  worked  up  into  solid  masses  by  high 
pressure  and  moderate  heat.2 

While  it  is  not  possible  as  a  usual  thing  to  mend  a  broken  object 
by  pressing  the  two  pieces  together  at  ordinary  temperatures, 
Spring3  has  shown  that  powders  may  be  welded  into  massive 
blocks  by  the  use  of  sufficiently  high  pressures.  This  means 
that  the  pressure  has  been  sufficient  to  squeeze  out  the  air  films 
coating  the  powders. 

Since  solids  will  coalesce  when  heated  and  subjected  to  pres- 
sure, there  is  no  reason  why  some  solid  powders  should  not  coa- 
lesce to  some  extent  under  the  influence  of  gravity  when  heated. 
This  is  known  as  sintering  or  sometimes  as  fritting.  The  two 
terms  are  not  distinguished  as  sharply  as  they  might  be.4  Sinter- 
ing should  be  used  when  a  powder  coalesces  without  melting  or 
with  but  slight  fusion,  so  that  the  product  is  still  granular  and 
more  or  less  porous.  Fritting  should  be  used  when  the  fusion  is 
quite  marked  and  the  product  is  distinctly  glassy.  The  forma- 

1  Scientific  Papers,  3,  523  (1902). 

2  ROLLER:  The  Utilization  of  Waste  Products,  62  (1915). 

3  Bull.  Acad.  roy.  belg.  (2)  49,  323  (1880);  Ann.  Chim.  Phys.  (5)  22,  170 
(1881);  Ber.  deutsch.  chem.  Ges.  16,  395  (1882);  Bull.  Soc.  chim.,  Paris, 
(2)  40,  520  (1883);  FRIEDEL:  Ibid.  (2)  39,  626;  40,  526  (1883);  JANNETZ, 
Neeland  Clermont:  Ibid.  (2)  40,  51  (1883);  Tammann:  Zeit.  Elektrochemie, 
15,  447  (1909). 

4  Percy's  Metallurgy:  Fuel,  46,  280  (1875). 


152  APPLIED  COLLOID  CHEMISTRY 

tion  of  bricks  and  pottery  involves  sintering;  the  materials  for 
glazes  are  fritted.  There  has  been  some  question  whether  sinter- 
ing could  take  place  without  at  least  incipient  fusion.  Lucas1 
maintained  that  it  was  a  property  of  amorphous  substances 
only,  in  which  case  it  would  be  nothing  more  than  the  slow  and 
partial  coalescence  of  very  viscous  drops.  Day  and  Allen2  found 
that  powdered  glasses  sinter  slowly  or  rapidly  several  hundred 
degrees  below  the  melting  temperature  of  crystals  of  the  same 
composition.  The  first  traces  of  sintering  appear  earlier  the 
finer  the  powder  and  the  slower  the  heating.  There  is  no  doubt 
that  the  sintering  of  powdered  glasses  is  due  to  flow  in  a  super- 
cooled liquid  and  that  it  is  a  phenomenon  of  viscosity  and  surface 
tension.  Powdered  crystalline  feldspars  do  not  sinter  readily 
below  their  melting  temperature  and,  at  first,  Day  and  Allen 
were  inclined  to  suspect  that  pure,  dry,  stable  crystals  would  not 
sinter  at  all.  Their  final  experiments,  however,  showed  that  the 
purest  crystalline  feldspar  which  they  could  prepare  still  sintered, 
though  very  slowly.  Kohlschutter3  found  that  porous  silver 
obtained  by  reducing  silver  salts  in  different  ways,  sintered  at 
temperatures  from  200°.  Instead  of  concluding  that  a  crystal- 
line substance  can  sinter,  he  drew  the  surprising  conclusion  that 
Lucas  was  right  and  that  therefore  the  degree  of  sintering  of  the 
silver  was  a  measure  of  the  amount  of  amorphous  silver  present, 
even  though  he  points  out  the  relation  between  the  porosity  and 
the  sintering. 

Other  things  being  equal  the  contraction  or  sintering  is  greater 
the  higher  the  temperature,  and  consequently  the  higher  the 
temperature  the  denser  the  solid.  Bricks  are  less  porous  the 
higher  the  temperature  of  burning.  Of  course  if  the  temperature 
is  reached  at  which  vitrification  takes  places  a  new  phenomenon 
occurs.  Magnesia  is  more  dense  the  higher  it  is  heated4  and  so 
is  lime.5  While  it  is  unquestionably  true  that  sintering  will 
take  place  less  readily  with  a  pure  crystalline  substance  than 
with  an  amorphous  one  or  with  one  containing  impurities  which 

1  Zeit.  phys.  Chem.,  62,  327  (1905). 

2  Carnegie  Inst.  Pub.,  31,  59  (1905);  Zeit.  phys.  Chem.,  64,  39  (1906). 

3  Liebig's  Ann.,  398,  37  (1913);  REEDY:  Am.  Jour.  Sci.  (4)  40,  407  (1915). 

4  DITTE:  Comptes  rendus,  73,  270  (1861);  MOISSAN:  Ibid.,  118,  506  (1894). 
6  WEIGHT:  Zeit.  anorg.  Chem.,  68,  397  (1910). 


COALESCENCE  153 

lower  the  melting-point1  and  cause  a  portion  of  the  mass  to 
soften  long  before  the  whole  mass  is  fused,  this  does  not  prevent 
sintering  occurring  with  perfectly  pure  crystalline  substances. 
The  technical  advantage  of  impurities  is  shown  in  the  case  of 
magnesia.2  The  crystalline  Styrian  magnesite  is  less  pure  than 
the  amorphous  Grecian  magnesite,  containing  more  iron.  It 
therefore  sinters  more  readily  and  at  a  lower  temperature.  It  is 
often  preferred  for  this  reason  to  the  purer  Grecian  magnesite, 
which  must  either  be  burned  at  a  higher  temperature  or  must  have 
some  bonding  material  added  which  will  cause  sintering  at 
lower  temperatures.  Of  course,  the  material  which  sinters  at  a 
low  temperature  will  not  stand  up  at  so  high  a  temperature  as  a 
purer  substance  which  has  been  shrunk  in  an  electric  furnace  at 
a  higher  temperature. 

The  sintering  of  tungsten3  and  of  tantalum4  powder  is  a  tech- 
nical process  carried  on  far  below  the  melting-point  of  tungsten, 
and  not  depending,  so  far  as  we  know,  on  the  presence  of  impuri- 
ties, though  these  may  be  present.  Another  interesting  case  of 
sintering  occurs  in  the  gold  assay.5  After  parting,  the  cornets 
are  of  a  dull  brown  color  and  very  friable.  When  annealed  at 
a  bright-red  heat  they  sinter  very  much  and  become  a  pure  gold- 
yellow  color.  This  has  been  studied  in  detail  by  Hanriot6  who 
took  an  alloy  containing  four  parts  of  silver  to  one  of  gold  and 
dissolved  the  silver  with  nitric  acid.  The  gold  was  left  in  a 
spongy  brown  form  which  decreased  rapidly  in  bulk  on  heating 
and  became  gold  color.  Wohler7  has  shown  that  the  glowing  of 
heated  chromic  oxide  jellies  is  the  result  of  rapid  sintering  and 
not  of  the  formation  of  an  allotropic  modification. 

Soot  becomes  sandy  if  heated  too  long  or  too  hot.  The  density 
of  red  phosphorus  depends  on  the  heat  treatment.  Many  pre- 
cipitates sinter  or  agglomerate  on  standing,  though  the  change  is 
not  usually  called  that.  The  agglomeration  is  often  accompanied 

1  ENDELL:  Silikat-Zeitschrift,  2,  1,  25  (1914). 
*HAVARD:  Refractories  and  Furnaces,  89  (1912). 
8  RUFF:  Zeit.  angew.  Chem.,  25,  1889  (1912). 
4  SIEMENS  and  HALSKE:  Jour.  Soc.  Chem.  Ind.,  23,  206  (1906). 
8  Percy's  Metallurgy:  Silver  and  Gold,  268  (1880). 

6Comptes  rendus,  151,  1355  (1910);  152,  215  (1911);  cf.  PRAT:  Ibid.,  70, 
841  (1870). 

7  Zeit.  Kolloidchemie,  11,  241  (1912). 


154  APPLIED  COLLOID  CHEMISTRY 

by  increased  resistance  to  chemical  action.  Strongly  heated 
lime  reacts  very  slowly  with  carbon  dioxide.1  After  short 
ignition,  silica  dissolves  in  a  boiling  solution  of  potassium  or 
sodium  carbonate,  but  does  not  if  ignited  for  a  long  time. 
Freshly  precipitated  arsenic  sulphide  gives  off  much  hydrogen 
sulphide  when  boiled  with  water;  but  does  not  after  being  heated- 
at  125°.  Pulverulent  iron  burns  in  the  air  while  massive  iron 
does  not.  The  action  of  alkali  on  hydroxides,  and  of  gelatine 
and  ammonia  or  potassium  bromide  on  silver  bromide  is  de- 
creased enormously  if  the  precipitates  are  allowed  to  stand  over 
night.  Holmes  and  Fall3  have  made  a  quantitative  study  of 
the  change  in  peptizability  of  ferric  arsenate,  so-called,  with  time 
of  standing. 

A  plastic  substance  is  one  which  can  be  moulded  by  pressure.4 
This  connotes  that  any  break  due  to  the  deformation  is  self- 
healing.  A  mobile  liquid  is  not  plastic,  because  it  will  not  retain 
its  shape.  With  increasing  viscosity  a  liquid  becomes  plastic. 
Molasses  candy  is  plastic  until  it  crystallizes.  Semifused  glass 
is  plastic.  The  time  factor  may  be  important.  Sealing-wax  is 
brittle  if  the  pressure  is  applied  rapidly;  but  is  plastic  if  the  rate 
of  deformation  is  low.  Since  metals  and  other  solids  coalesce 
under  high  pressure,  there  is  no  reason  why  they  should  not  be 
plastic  under  the  same  conditions,  as  indeed  they  are.  We  make 
use  of  this  property  when  squirting  metals  into  rods  or  pipes. 
Under  high  pressure  ice  is  also  plastic.  It  has  been  claimed  by 
Spring5  and  by  Kahlbaum6  that  solids  become  almost  like  mobile 
fluids  under  high  pressure;  but  it  seems  to  have  been  proved 
conclusively  by  Spezia7  that  this  is  not  true  when  the  pressure 
is  uniform  and  that  the  apparent  fluidity  occurs  only  when  there 
is  a  shearing  force.  The  question  of  unequal  pressures  has  been 

1  RAOULT:  Comptes  rendus,  92,  189  (1881);  SOSMAN,  HOSTETTER  and 
MERWIN:  Jour.  Washington  Acad.  Sci.,  5,  563  (1915). 

SRAMMEL,SBERG:  Jour.  Chem.  Soc.,  26,  242  (1873);  cf.  LAUFER:  Ber. 
deutsch.  chem.  Ges.,  11,  60,  935  (1878). 

s  Jour.  Am.  Chem.  Soc.,  41,  713  (1919). 

4  DUFF:  A  Text-Book  of  Physics,  4th  Ed.,  119  (1916). 

6  Bull.  Acad.  Soc.  belg.  (2)  49,  364  (1880). 

6  Verh.  Naturforscherges.  Basel,  15,  14  (1903). 

7  Atti.  Accad.  Sci.  Torino,  46,  525  (1911);  46,  682  (1912). 


COALESCENCE  155 

discussed  in  some  detail  by  Johnston.1  Iron  filings  in  presence 
of  a  magnet  might  be  considered  a  special  case  of  plasticity, 
though  not  one  of  any  great  interest.  Another  special  case  of 
partial  plasticity  is  the  mineral  itacolumite2  which  is  said  to  have 
something  akin  to  a  series  of  ball-and-socket  joints.  Each  ball 
and  socket  does  not  admit  of  a  great  play,  and  is  not  smooth 
and  perfect  like  that  of  the  joint  of  a  limb.  The  stone  is  built 
up  of  grains  and  masses  of  grains,  which  are  loosely  coherent  and 
form  irregular  cavities  in  which  are  engaged  projecting  parts  of 
other  grains  or  masses  of  grains  of  sand,  susceptible  of  a  slight 
motion  in  the  cavity,  in  some  cases  in  one  direction,  and  in  others 
in  several  or  in  all  directions.  This  freedom  of  motion  is  of  the 
true  quality  of  a  ball-and-socket  joint.  Perhaps  it  would  be 
better  to  call  this  mineral  flexible  rather  than  plastic. 

Between  the  extreme  cases  of  a  solid  such  as  a  metal  and  a 
liquid  such  as  glass  or  molasses  candy,  we  have  the  intermediate 
case  of  putty  which  is  whiting  and  oil,  or  a  solid  with  a  liquid 
film  around  it.  If  a  liquid  is  adsorbed  strongly  by  a  solid,  thin 
films  of  the  liquid  will  hold  the  solid  particles  together,  while 
still  permitting  them  to  move  relatively  to  one  another.  The 
tendency  of  liquid  surfaces  to  coalesce  causes  any  break  to  heal. 
A  liquid  film  may  therefore  act  as  a  bond  for  solids  and  may  make 
the  mass  plastic.  Oil  is  used  as  a  binder  in  roads  and  to  make 
putty  out  of  calcium  carbonate.  Everybody  knows  that  dry 
sand  cannot  be  moulded  whereas  wet  sand  can.  Fondant  consists 
of  very  small  crystals  of  sugar  made  plastic  by  a  syrupy  film  of 
invert  sugar  and  water.  Plastic  dynamite  is  made  by  adding 
three  parts  of  nitroglycerine3  to  one  part  of  diatomaceous  earth. 
Bingham4  has  made  a  preliminary  study  of  the  concentration  at 
which  plasticity  begins  or  ends.  If  a  finely  powdered  solid  is 
added  to  a  liquid,  the  viscosity  of  the  liquid  is  increased  or  the 
fluidity,  which  is  the  reciprocal  of  the  viscosity,  is  decreased. 
The  fluidity  was  calculated  from  the  rate  of  flow  of  the  liquid 
through  a  viscosimeter.  At  temperatures  between  25°  and  60° 

1  Jour.  Am.  Chem.  Soc.,  34,  788  (1912);  JOHNSTON  and  ADAMS:  Am.  Jour. 
Sci.  (4)  40,  351  (1915). 

2  WETHERILL:  Chem.  News,  22,  266  (1870). 

3  THORP:  Outlines  of  Industrial  Chemistry,  3d  Ed.,  482  (1916). 

4  Am.  Chem.  Jour.,  46,  278  (1911);  Jour.  Franklin  Inst.,  181,  845  (1916). 


156  APPLIED  COLLOID  CHEMISTRY 

the  fluidity  dropped  to  zero  at  the  same  concentration,  so  the 
concentration  for  zero  fluidity  is  independent  of  the  temperature 
over  the  range  studied.  With  infusorial  earth  in  water,  zero 
fluidity  was  reached  at  a  volume  concentration  of  about  13 
percent  solids;  with  China  clay  at  about  4  percent;  with  the 
graphite  used  in  Acheson's  aquadag  zero  fluidity  was  reached  at 
a  volume  concentration  of  about  5.5  percent  graphite;  and  with 
an  unspecified  clay  at  about  19.5  percent  clay.  With  infusorial 
earth  in  alcohol,  zero  fluidity  was  reached  at  a  volume  concentra- 
tion of  about  12  percent  solids.  The  mixtures  having  zero 
fluidity  are  not  stiff  and  will  not  maintain  their  shape.  At 
higher  concentrations  there  is  a  change  from  viscous  flow  to 
plastic  flow.  The  distinction  made  by  Bingham  is  that  with 
viscous  flow  any  shearing  force — no  matter  how  small — will 
produce  permanent  deformation,  whereas,  in  the  case  of  plastic 
flow,  it  is  necessary  to  use  a  shearing  force  of  finite  magnitude  in 
order  to  produce  a  permanent  deformation.  It  seems  reasonable 
to  assume  that  we  reach  zero  fluidity  when  liquid  enough  is  added 
to  the  solid  to  begin  to  scatter  the  particles,  in  other  words,  when 
about  enough  liquid  is  added  to  fill  the  voids.  This  is  true  in 
one  case  studied  by  Bingham.  The  clay  referred  to  contained 
81.6  percent  voids  and  required  80.5  volume  percent  of  water 
to  bring  it  to  zero  fluidity.  The  two  values  would  only  be 
identical  in  case  the  volume  of  the  adsorbed  water  just  equalled 
the  volume  of  the  adsorbed  air.  Of  course  this  very  important 
generalization  of  Bingham ;s  must  be  tested  in  more  cases  before 
it  can  be  considered  as  definitely  established;  but  it  is  so  obvious, 
after  it  has  been  pointed  out,  that  it  must  be  approximately  true. 
This  discovery  of  Bingham's  may  be  of  distinct  importance  in  the 
paint  industry.  The  oil  requirement  for  a  given  pigment  is  a 
very  arbitrary  amount  and  experts  often  differ  widely  in  their 
values.  Reproducible  figures  could  be  obtained  if  in  each  case 
there  was  determined  the  amount  of  oil  necessary  to  give  zero 
fluidity.  Since  this  is  more  oil  than  painters  wish  to  use,  it 
might  be  advisable  to  adopt  as  the  standard,  some  definite  frac- 
tion of  the  amount  of  oil  necessary  to  produce  zero  fluidity. 

Emley1  has  made  some  determinations  on  the  viscosity  of 
lime  pastes.  Judging  from  the  diagram  given,  the  viscosity 

1  Trans.  Am.  Ceramic  Soc.,  15,  401  (1913). 


COALESCENCE  157 

would  become  infinite  or  the  fluidity  zero  when  there  was  about 
50  percent  of  hydrated  lime  in  the  paste.  Since  Emley  used 
the  Clark  viscosimeter,  one  does  not  know  to  what  extent  his 
results  are  comparable  with  those  of  Bingham. 

Since  the  concentration  producing  zero  fluidity  apparently 
depends  on  the  volume  of  the  voids,  it  is  necessary  to  consider 
approximately  what  this  may  be.  If  equal  spheres  are  taken 
and  are  piled  so  that  each  sphere  is  directly  on  top  of  another — 
minimum  or  cubical  piling — the  voids  constitute  theoretically 
about  48  percent  of  the  whole,  irrespective  of  the  size  of  the 
spheres.  If  the  spheres  are  piled  so  that  each  one  rests  equally 
on  the  four  in  the  layer  below — maximum  or  hexagonal  piling — 
the  theoretical  percentage  of  voids  is  nearly  26,  again  irrespective 
of  the  size  of  the  spheres.  With  solids  of  different  sizes  and 
different  shapes,  the  voids  may  theoretically  be  made  almost 
anything.  If  the  solids  have  condensed  air  films  round  them  so 
that  they  do  not  come  actually  in  contact,  the  percentage  of 
voids  will  go  up  with  decreasing  size  of  the  solids.  As  a  matter 
of  fact,  we  have  seen  that  the  voids  in  carbon  black  may  run  up 
to  95  percent  or  thereabouts.  If  the  air-adsorption  were  negli- 
gible and  if  the  particles  were  cubes  of  the  same  size  and  arranged 
in  an  orderly  fashion  the  voids  would  be  practically  zero.  As  a 
matter  of  fact  round-grained  water-worn  sands  have  2-5  percent 
less  voids  than  corresponding  sharp  grains  of  sand1  because  the 
former  pack  better.  The  difference  between  the  theoretical 
values  and  the  actual  values  as  found  under  certain  experimental 
conditions  is  shown  by  the  statement  that  large  enough  masses 
of  equal-sized  marbles  cannot  be  poured  or  tamped  into  a  large 
vessel  so  as  to  give  less  than  44  percent  voids.  Undoubtedly 
these  would  have  jarred  down  in  time  so  as  to  occupy  a  lesser 
volume;  because  this  result  comes  very  close  to  that  required  for 
cubical  piling.  The  effect  of  moisture  on  sand  is  also  unexpected 
at  first.2  Moist  sand  occupies  more  space  and  weighs  less  per 
cubic  foot  than  dry  sand.  The  explanation  of  this  apparent 
anomaly  is  that  a  film  of  water  coats  each  grain  of  sand  and 
separates  it  from  the  adjacent  ones.  Since  fine  sand  has  a  large 
number  of  grains  and  consequently  more  surface  area,  addition 

1  TAYLOR  and  THOMPSON:  Plain  and  Reinforced  Concrete,  174  (1912). 

2  TAYLOR  and  THOMPSON:  Plain  and  Reinforced  Concrete,  176  (1912). 


158  APPLIED  COLLOID  CHEMISTRY 

of  water  to  it  causes  a  greater  increase  in  bulk  than  in  the  case 
of  coarse  sand.  If  a  small  quantity  of  water  is  poured  into  a 
vessel  containing  dry  sand,  the  friction  of  the  particles  keeps  the 
volume  from  increasing;  but  the  bulk  increases  if  the  moistened 
sand  is  dumped  out  and  then  turned  back  into  the  vessel  with  a 
shovel  or  trowel.  The  percentage  of  water  by  weight  which  will 
give  the  greatest  volume — corresponding  to  the  largest  percent- 
age of  voids — varies  with  different  sands  from  5  to  8  percent. 

Since  the  densest  mixture  occurs  with  particles  of  different  sizes, 
it  is  important,  in  making  concrete,  to  grade  the  materials  so  as 
to  get  the  maximum  density.1  On  the  other  hand  for  a  sand 
filter  the  grains  of  sand  should  be  of  approximately  the  same  size,2 
as  otherwise  the  filter  clogs  too  readily.  The  question  has  also 
been  raised  but  not  answered,  as  to  what  are  the  proper  sizes  for 
the  balls  in  the  ball  mill.  Since  there  should  probably  be  more 
of  the  small  balls  than  the  large  ones,  the  proper  ratio  cannot  be 
obtained  if  only  large  balls  are  added  as  the  others  wear  down.3 

A  number  of  curious  things  in  regard  to  wetted  sand  have  been 
pointed  out  by  Osborne  Reynolds.4  If  we  have  sand  in  maxi- 
mum piling  in  a  rubber  bag  and  enough  water  to  form  a  thin 
layer  over  the  surface,  squeezing  the  sand  will  necessarily  cause 
it  to  expand  or  dilate,  and  the  water  will  be  sucked  down  into  the 
dilated  mass.  We  are  familiar  with  this  experiment  in  another 
form.  The  wet  sand  below  high  water  mark  is  in  the  position  of 
maximum  piling.  When  one  walks  over  this  sand,  the  sand 
whitens  momentarily  and  appears  to  dry  around  the  foot.  The 
sand  is  full  of  water,  the  surface  of  which  is  kept  up  to  that  of  the 
sand  by  capillary  attraction.  The  pressure  of  the  foot  dilates 
the  sand  and  since  the  water  does  not  suck  up  rapidly  enough, 
the  water  level  is  lower  in  the  dilated  sand  than  before  and  conse- 
quently the  sand  appears  dryer  until  more  water  has  been  brought 
up  from  below  by  capillary  action.  On  raising  the  foot,  the 
sand  in  and  around  the  foot-print  usually  becomes  wet  momen- 
tarily, because  the  sand  contracts  when  the  shearing  pressure  is 
taken  off  and  the  excess  of  water  is  left  momentarily  on  the  surf  ace. 

1  UPTON:  Materials  of  Construction,  304  (1916). 

2  MENDELEEFF:  Jour.  Chem.  Soc.,  48,  114;  LUEGER:  Ibid.  48,  198  (1885). 

3  Chem.  Met.  Eng.,  14,  285,  292  (1916). 

*  Phil.  Mag.  (5)  20,  469  (1885);  Scientific  Papers,  2,  217  (1901). 


COALESCENCE  159 

In  a  filter  press  the  solid  to  be  filtered  tends  to  go  into  cubical  pil- 
ing because  that  lets  the  water  through  most  readily.  If  the  crys- 
tals are  displaced  so  that  they  pass  into  hexagonal  piling,  water  will 
be  set  free.  Hatschek1  says  in  regard  to  this  that  it  is  a  perfectly 
familiar  fact  that  apparently  dry  cakes  of  calcium  sulphate,  for 
instance,  which  break  with  a  clean  fracture  can  be  reduced  to 
an  almost  liquid  mass  by  mere  shaking  It  seems  probable 
that  in  quicksands  the  sand  is  in  a  state  of  cubical  piling,  making 
it  possible  for  a  man  to  sink  in  it  and  raise  the  water  without 
raising  the  sand. 

To  show  some  of  the  phenomena  due  to  dilatancy  Osborne 
Reynolds  took  a  thin,  transparent,  india-rubber  balloon,  filled  it 
with  about  six  pints  of  sand  and  water,  and  closed  it  so  that  it 
contained  no  air.  There  was  more  than  enough  water  to  fill  the 
voids  when  the  sand  was  at  maximum  density  but  not  enough  to 
fill  them  when  the  sand  was  at  minimum  density.  When  this 
rubber  sack  was  laid  on  the  table,  the  elasticity  of  the  rubber 
gave  it  a  rounded  shape.  The  sand  settled  into  the  condition  of 
maximum  piling  and  there  was  consequently  a  layer  of  water 
above  the  surface  of  the  sand.  The  bag  can  be  squeezed  and 
altered  in  shape  as  though  it  had  no  firmness,  so  long  as  the 
sand  does  not  rise  to  the  surface  of  the  water.  If  the  bag  is 
placed  between  two  vertical  boards  and  squeezed,  it  changes 
shape  at  first  apparently  without  resistance,  but  soon  comes  to  a 
dead  stop.  Turning  it  on  its  side  and  laying  a  56  pound  weight 
on  it  produced  no  further  change  in  shape;  but  on  removing  the 
weight  the  bag  goes  back  to  its  almost  rounded  shape.  If  the 
bag  is  shaken  slightly  while  it  is  being  squeezed,  the  sand  keeps 
at  its  densest  and  the  bag  can  be  pressed  out  until  it  is  a  broad 
flat  plate.  The  bag  is  soft  so  long  as  it  is  squeezed;  but  if  the 
pressure  is  removed,  the  elasticity  of  the  rubber  tends  to  draw 
the  bag  back  to  its  rounded  form.  This  dilates  the  sand  and 
causes  the  excess  of  water  to  be  sucked  into  the  pores.  So  soon 
as  there  is  no  excess  of  water,  no  further  change  takes  place  and 
the  bag  remains  a  flat  cake  with  peculiar  properties.  It  yields 
at  once  to  pressure  on  its  sides,  such  pressure  having  nothing  but 
the  elasticity  of  the  bag  to  overcome1  because  pressure  in  that 
direction  causes  the  sand  to  contract.  It  is  perfectly  rigid, 

1  Jour.  Soc.  Chem.  Ind.,  21,  540  (1908). 


160  APPLIED  COLLOID  CHEMISTRY 

however,  to  radial  pressures  because  such  pressures  tend  to 
dilate  the  sand  still  farther.  When  placed  on  its  edge,  it  will 
hold  up  a  hundred  pounds.  If,  however,  it  is  pressed  sufficiently 
on  the  sides  while  holding  up  such  a  weight,  all  the  strength 
vanishes  and  it  is  again  a  rounded  bag  of  loose  sand  and  water. 
By  shaking  the  bag  into  a  mould,  it  can  be  made  to  take  any 
shape;  then  by  drawing  off  the  excess  of  water  and  closing  the  bag, 
the  sand  becomes  perfectly  rigid  and  will  not  change  its  shape 
unless  the  bag  is  torn,  Since  the  sand  is  at  its  maximum  density, 
no  amount  of  shaking  will  produce  a  change.  In  this  way  bricks 
can  be  made  of  sand  or  fine  shot  with  water  in  the  thinnest  india- 
rubber  envelope  and  they  will  withstand  as  much  pressure  as 
an  ordinary  brick  without  change  of  shape. 

The  clay  workers  use  the  word  plasticity  in  a  special  sense  to 
denote  that  a  clay  is  plastic  and  will  burn  to  a  coherent  mass. 
From  this  point  of  view  wet  sand  is  not  plastic  because  it  falls 
to  pieces  when  dried.  To  meet  this  definition  we  must  have  in 
the  water  some  bonding  material  presumably  in  a  gelatinous 
form.  The  film  of  water  about  the  clay  particles  is  usually 
assumed  to  be  50^  in  thickness.1  There  is  no  difficulty  in 
accounting  for  a  gelatinous  film.2  Cushman3  has  shown  that  the 
cementing  power  of  rock  powders  is  due  to  the  formation  of  gela- 
tinous silica,  ferric,  oxide,  etc.  If  we  have  a  gelatinous  film  which 
is  adsorbed  strongly  by  the  solid  particles  and  which  can  take 
up  and  lose  water,  we  shall  have  plasticity.  In  the  case  of  clay 
the  gelatinous  material  may  be  hydrous  alumina,  hydrous  silica, 
or  some  intermediate  composition.  The  difficulty,  however,  is  to 
account  for  its  remaining  plastic  after  moderate  drying  and 
for  its  readsorbing  water  to  form  a  gelatinous  mass.  This  is 
probably  due  to  the  presence  of  some  salt;  but  we  do  not  know 
what  salt  and  consequently  we  cannot  tell  why  one  clay  is  plastic 
and  another  not,  and  we  do  not  know  what  we  should  add  to  the 
non-plastic  clay  to  make  it  plastic.  People  have  never  studied 
the  problem  from  a  rational  point  of  view. 

From  this  study  of  coalescence  we  conclude  that  liquid  drops 

1  GROUT  and  POPPE:  Trans.  Am.  Ceramic  Soc.,  14,  73  (1912);  DAVIS: 
Ibid.,  16,  19  (1914). 

2QuiNCKE:  Drude's  Ann.,  7,  74  (1902). 
3  Jour.  Am.  Chem.  Soc.,  25,  451  (1903). 


COALESCENCE    '  161 

or  solid  grains  will  coalesce  or  agglomerate  if  brought  into  actual 
contact.  To  keep  them  from  doing  so  we  must  have  a  surface 
film  of  some  sort.  Since  this  coalescence  or  agglomeration  takes 
place  spontaneously,  work  must  be  done  on  the  liquid  or  solid 
to  disintegrate  it.  It  is  easy  to  see  that  a  film  might  keep  drops 
separated  and  yet  not  break  up  a  large  drop  into  a  lot  of  smaller 
ones.  Dirty  mercury  will  remain  indefinitely  as  a  single  liquid 
mass  so  far  as  we  know.  If  we  disintegrate  the  mass  by  shaking, 
a  film  will  form  over  each  drop  and  will  keep  the  drops  from  coa- 
lescing. This  "flouring"  of  mercury,  as  it  is  called,  is  a  distinct 
source  of  trouble  when  extracting  gold  by  the  amalgamation 
process.  In  most  cases  the  film  is  oxide,  chloride,  or  grease, 
though  it  may  consist  of  oxides  of  other  metals  if  one  starts  with  an 
amalgam .  Anything  that  will  dissolve  the  surface  film  will  cause  the 
drops  of  mercury  to  run  together.  Treatment  with  potassium 
cyanide  is  therefore  very  effective.  Emulsions  of  oil  in  water 
are  obtained  if  some  substance  like  sodium  oleate  is  present  to 
form  a  film  over  the  drops  of  oil,  thus  keeping  them  from  coales- 
cing.1 It  is  easy  to  see  that  the  work  necessary  to  break  up  a 
liquid  or  solid  mass  into  drops  or  grains  will  be  less  if  we  add  some- 
thing to  the  solution  which  will  lower  the  surface  tension  of  the 
liquid  or  solid.  In  some  cases  water  alone  will  cause  the  disinte- 
gration. With  gelatine  we  get  reversible  disintegration  at  very 
moderate  temperatures,  say  30°;  but  cellulose  requires  a  very 
much  higher  temperature.  We  do  not  know  at  all  to  what  extent 
this  matter  of  disintegration  depends  upon  the  structure;  but  a 
single  illustration  from  the  field  of  true  solutions  will  show  the 
possibility  of  such  a  factor.  If  we  place  a  crystal  of  rock  candy 
in  cold  water  the  crystal  dissolves  so  as  to  keep  its  shape  approxi- 
mately. It  grows  smaller  but  does  not  fall  to  pieces.  If  we 
put  a  lump  of  sugar  into  cold  water,  the  smaller  crystals  which 
act  as  binding  material  dissolve  first  and  the  lump  disintegrates 
to  a  mass  of  grains. 

TYPES  OF  PRECIPITATES 

Sufficiently  finely  divided  particles  will  be  kept  in  suspension 
in  a  liquid  indefinitely  by  the  Brownian  movements,  provided 
coalescence  and  the  resulting  agglomeration  are  prevented. 

1  Cf.  DONNAN  and  POTTS:  Zeit.  Kolloidchemie,  7,  208  (1910). 
11 


162  APPLIED  COLLOID  CHEMISTRY 

Such  a  colloidal  solution  is  called  a  sol.  One  may  use  the  words 
hydrosol,  alcosol,  etc.,  to  show  that  the  dispersing  liquid  is 
water,  alcohol,  etc.  When  the  colloidal  particles  agglomerate 
and  precipitate,  the  precipitate  is  called  a  gel,  or  a  hydrogel  in 
case  the  dispersing  liquid  is  water.  The  word  gel  is  also  used  to 
include  a  jelly,  which  differs  from  a  gelatinous  precipitate  in 
that  there  is  no  supernatant  liquid  when  first  formed.  It  is  really 
the  general  term  for  a  solid  or  semi-solid  colloid.  The  change 
from  a  gel  to  a  sol  is  usually  called  peptization,  a  term  which  we 
owe  to  Graham.  The  reverse  change  from  sol  to  gel  was  called 
pectization  by  Graham;  but  this  term  has  never  become  popular. 
A  stable  colloidal  solution  will  consist  of  very  fine  particles  kept 
from  agglomerating  by  a  protecting  film1  of  some  sort,  electrifica- 
tion being  equivalent  to  a  film.2  Small  particles3  may  be  ob- 
tained either  by  condensation  from  vapor  or  solution  (usually 
called  precipitation)  or  by  disintegration  of  larger  masses.  The 
disintegration  is  often  called  peptization  especially  when  done 
chemically,  as  opposed  to  mechanical  or  electrical  disintegration. 
People  have  distinguished  eight  types  of  precipitates:  crystalline; 
colloidal  or  non-settling;  flocculent;  curdy;  sandy;  gelatinous; 
jelly-like;  fluid.  A  crystalline  precipitate  is  one  that  is  distinctly 
crystalline,  and  settles  rapidly,  as  when  KOH  is  neutralized  by 
H2SO4  or  when  moderately  concentrated  solutions  of  lead  nitrate 
and  potassium  chromate  are  mixed.  A  non-settling  precipitate 
may  be  anhydrous  or  hydrous;  there  is  little  or  no  agglomeration. 
A  typical  case  is  the  precipitation  of  lead  chromate  from  very 
dilute  solutions  of  lead  nitrate  and  potassium  chromate.4  A 
flocculent  precipitate  is  obtained  when  a  non-settling,  non-gela- 
tinous precipitate  agglomerates  to  a  bulky  mass.  It  is  easy  to 
obtain  a  flocculent  precipitate  of  silver  chloride  from  suitable 
solutions  of  silver  nitrate  and  potassium  chloride.  Haber  and 
Oordt5  call  the  precipitate  of  beryllium  hydroxide  flocculent;  but 
I  am  not  certain  whether  they  are  limiting  the  term  sufficiently 
sharply.  A  curdy  precipitate  is  obtained  when  a  flocculent 

1  Cf.  HOBER:  Physikalische  Chemie  der  Zelle  und  der  Gewebe,  233  (1914). 
2Cf.  HARDY:  Proc.  Roy.  Soc.,  86A,  601  (1912). 

3  FREUNDLICH:  Kapillarchemie,  527  (1909). 

4  FREE:  Jour.  Phys.  Chem.,  13,  114  (1909). 

5  Zeit.  anorg.  Chem.,  38,  377  (1904). 


COALESCENCE  163 

precipitate  agglomerates  to  a  fairly  dense  mass.  The  typical 
case  is,  of  course,  the  precipitation  of  caseine  by  acids,  but  a 
similar  precipitate  can  be  obtained  with  AgNO3  and  HC1. 
There  is  no  sharp  dividing  line  between  a  flocculent  and  a  curdy 
precipitate.  A  sandy  precipitate  is  obtained  when  a  flocculent 
or  a  curdy  precipitate  agglomerates  to  a  dense  mass.  The  alumina 
from  hot  sodium  aluminate  in  the  Bayer  process  is  a  sandy 
precipitate.1  A  granular  or  sandy  alumina  is  said  to  be  precipi- 
tated by  ammonia2  at  66°.  A  gelatinous  precipitate  is  always 
viscous;  it  involves  agglomeration  to  a  bulky  form.  Hydrous 
alumina  and  hydrous  ferric  oxide,  when  precipitated  cold,  are 
typical  gelatinous  precipitates.  When  a  sparingly  soluble  salt  is 
precipitated  from  very  concentrated  solutions  we  apparently 
always  get  a  gelatinous  precipitate.  Thus  gelatinous  barium  sul- 
phate is  obtained  by  mixing  concentrated  solutions  of  barium 
sulphocyanate  and  manganese  sulphate,3  and  gelatinous  calcium 
carbonate  by  mixing  concentrated  solutions  of  calcium  chloride 
and  sodium  carbonate.  The  difference  between  a  gelatinous 
precipitate  and  a  jelly  is  that  the  liquid  phase  is  visible  as  such 
in  the  case  of  the  gelatinous  precipitate  and  is  not  in  the  case  of 
the  jelly.  A  jelly  is  always  hydrous,  viscous,  and  bulky;  it  may 
consist  of  a  number  of  coalescing  drops  or  rods,  in  which  case 
both  phases  are  continuous;  or  it  may  consist  of  coalescing  films,  in 
which  case  we  have  a  honeycomb  structure,  with  water  as  the 
internal  phase.  A  typical  jelly  can  be  obtained  with  sodium 
silicate  and  hydrochloric  acid.  A  fluid  precipitate  is  one  which 
is  too  mobile  to  be  classed  as  a  gelatinous  precipitate.  Sodium 
phenolate  or  sodium  oleate  with  an  excess  of  HC1  gives  a  fluid 
precipitate. 

The  mean  mass  of  the  crystals  in  a  crystalline  precipitate  is 
given  by  the  total  weight  of  the  precipitate  divided  by  the  number 
of  crystals.  Experimentally  we  find  that  the  number  of  nuclei 
increases  less  rapidly  than  the  concentration  at  first  and  more 
rapidly  later.  When  very  dilute  solutions  are  mixed,  the  number 
of  nuclei  is  so  large  relatively  to  the  weight  of  the  precipitate 
that  the  particles  themselves  are  very  small.  With  increasing 

1  Russ:  Zeit.  anorg.  Chem.,  41,  216  (1904). 

2  VON  WEIMAEN:  Griindziige  der  Dispersoidchemie,  67  (1911). 

3  TAYLOR:  Chem.  News,  103,  169  (1911). 


164  APPLIED  COLLOID  CHEMISTRY 

concentration  we  get  increasing  size  of  crystals  for  a  while  and 
then  decreasing  size,  because  the  number  of  nuclei  then  increases 
more  rapidly  than  the  concentration.  At  very  high  concentra- 
tions a  gelatinous  precipitate  is  always  obtained,  as  has  been 
pointed  out.  It  is  possible  that  we  have  here  the  appearance  of 
a  viscous  and  instable  second  liquid  layer;  but  this  has  not  yet 
been  proved.  According  to  Clowes  a  jelly  is  formed  when  a 
viscous  phase  separates  slowly  in  presence  of  a  substance  which 
tends  to  keep  it  dispersed.1  To  obtain  large  crystals  one  must 
have  a  solution  so  slightly  supersaturated  that  no  nuclei  form 
spontaneously  and  that  no  branched  crystals  form  in  presence 
of  a  single  crystal.2  Von  Weimarn3  cites  the  case  of  a  spring  in 
which  crystals  of  barium  sulphate  had  grown  5  cm  long !  Geolo- 
gists have  prepared  certain  substances  in  distinct  crystals  by 
arranging  for  the  slow  diffusion  of  dilute  solutions  of  two  salts 
which  form  the  desired  substance  by  metathetical  reaction. 
Johnston4  has  improved  on  the  technique  by  allowing  the  two 
solutions  to  diffuse  into  a  large  vessel  containing  water,  causing  a 
further  dilution.  Dreaper5  obtained  distinct  crystals  by  letting 
solutions  diffuse  through  sand,  the  rate  of  diffusion  being  rela- 
tively slow  on  account  of  the  capillary  spaces.  Holmes6  ob- 
tained crystals  of  silver  bichromate  in  flat  needles  one  cm  long 
by  this  method,  substituting  aluminum  powder  for  sand.  He 
filled  a  small  test-tube  full  of  N/10  potassium  iodide,  covered  the 
mouth  of  the  tube  with  gold-beater's  skin,  and  immersed  the 
tube  in  a  small  beaker  containing  a  saturated  lead  acetate  solu- 
tion. At  once  an  almost  amorphous  precipitate  of  lead  iodide 
appeared  on  the  under  side  of  the  membrane  and  in  less  than  a 
minute  crystals  of  lead  iodide  fell  in  a  gleaming  shower  to  the 
bottom  of  the  test-tube.  If  the  same  solutions  are  mixed  in  a 

*Cf.  HARDY:  Jour.  Physiology,  24,  167  (1899);  Proc.  Roy.  Soc.,  66,  95 
(1900);  87 A,  29  (1912);  HOLMES  and  RINDFUSZ:  Jour.  Am.  Chem.  Soc., 
38,  1970  (1916);  HOLMES  and  FALL:  Ibid.,  41,  763  (1919). 

2Cf.  HOPKINS:  Am.  Chem.  Jour.,  25,  413  (1907);  BOWMAN:  Jour.  Soc. 
Chem.  Ind.,  26,  143  (1906);  HOSTETTER:  Jour.  Washington  Acad.  Sci.,  9, 
85  (1919);  MOORE:  Jour.  Am.  Chem.  Soc.,  41,  1060  (1919). 

3  Griindziige  der  Dispersoidchemie,  55  (1911). 

4  Jour.  Am.  Chem.  Soc.,  36,  16  (1914). 
6  Jour.  Soc.  Chem.  Ind.,  32,  678  (1913) 
6  Jour.  Phys.  Chem.,  21,  709  (1917). 


COALESCENCE  165 

test-tube  without  using  a  membrane,  a  yellow  powder  is  obtained 
and  no  easily  recognized  crystals.  This  method  as  developed  by 
Holmes  seems  the  most  promising  of  any  because  it  appears  to 
give  results  with  more  concentrated  solutions  and  would  pre- 
sumably work  even  better  with  more  dilute  solutions.  Excessive 
supersaturation  and  consequently  the  formation  of  very  fine 
precipitates  can  be  obtained  by  using  a  solvent  in  which  the 
resulting  salt  is  practically  insoluble.1  It  is  easier  to  avoid 
supersaturation  when  the  substance  is  more  soluble.  Thus 
one  can  get  crystals  of  silver  chloride  without  trouble  from  an 
ammoniacal  solution.2  The  effect  of  othefr  substances  in  the  solu- 
tion depends  on  their  effect  on  the  supersaturation  and  whether 
they  are  adsorbed  by  the  precipitate. 

If  a  precipitate  forms  in  the  presence  of  a  substance  which  is 
adsorbed  strongly  by  it,  the  adsorption  will  prevent  the  growth 
of  the  crystals  and  the  precipitate  will  therefore  come  down  in  a 
finer  state  of  subdivision.  A  typical  case  is  the  precipitation  of 
silver  bromide  in  presence  of  gelatine.  The  addition  agents  in 
electrolytic  refining  are  substances  which  move  to  the  cathode 
under  the  influence  of  the  current  and  which  are  adsorbed 
strongly  by  the  metal  as  it  comes  out.  Barium  sulphate  precipi- 
tates in  a  finely  divided  form  in  presence  of  barium  chloride3  or 
potassium  sulphate,  both  of  which  it  adsorbs,  and  more  coarsely 
crystalline  in  presence  of  sulphuric  acid  or  hydrochloric  acid.4 
Barium  sulphate  shows  selective  adsorption  for  its  own  ions, 
which  accounts  for  the  behavior  of  barium  chloride5  and  potas- 
sium sulphate.  The  peptizing  action  of  sulphate  ions  in  sulphuric 
acid  is  cut  down  by  the  tendency  of  hydrogen  ion  to  be  adsorbed 
strongly.  The  precipitate  is  therefore  more  coarsely  crystalline. 
The  solvent  action  of  hydrochloric  acid  is  a  factor  in  making  the 
barium  sulphate  precipitate  more  coarsely  crystalline.6  The 
question  has  been  raised  whether  the  finest  precipitates  are 
crystalline  or  amorphous.  Von  Weimarn7  calls  them  all  crystal- 

1  BYK:  Zeit.  Elektrochemie,  13,  38  (1907). 

2  REINDERS:  Zeit.  phys.  Chem.,  77,  696  (1911). 

3  FOULK:  Jour.  Am.  Chem.  Soc.,  18,  793  (1896). 
*  MAR:  Jour.  Chem.  Soc.,  60,  1137  (1891). 

6  WEISER:  Jour.  Phys.  Chem.,  21,  314  (1917). 

6  See  also  OSBORNE:  Jour.  Phys.  Chem.,  17,  629  (1913). 

7  Zeit.  Kolloidchemie,  3,  166  (1908), 


166  APPLIED  COLLOID  CHEMISTRY 

line;  but  he  considers  that  liquids  and  gases  are  crystalline, 
which  throws  him  out  of  court,  because  he  is  not  using  the  word 
in  the  same  sense  as  other  people.  A  supercooled  liquid  like 
glass  is  amorphous  in  the  ordinary  sense  of  the  term.  Whether  a 
precipitate  is  amorphous  or  not  depends  upon  its  being  a  super- 
cooled liquid  or  not.  Von  Weimarn  is  undoubtedly  right  in 
saying  that  many  precipitates  are  really  crystalline,  though  they 
are  classed  as  amorphous;  but  there  is  no  question  that  there  are 
amorphous  precipitates. 

In  many  cases  there  is  a  gradual  change  from  an  amorphous 
precipitate  to  a  crystalline  one  on  standing1  and  still  more  on 
heating.2  At  higher  temperatures  precipitates  are  less  hydrous 
and  tend  to  be  more  coarsely  crystalline.  There  are  no  data  in 
regard  to  a  substance  like  sodium  sulphate  which  is  less  soluble 
at  80°  than  at  40°.  When  barium  sulphate  is  precipitated  hot  it 
does  not  run  through  the  filter  so  readily  as  when  precipitated 
cold.  Other  conditions  being  the  same,  a  non-settling  silver3  is 
obtained  by  reduction  with  hydrazine  at  16°  and  pulverulent 
silver  by  reduction  at  80°  to  90°. 

THEORY  OF  PEPTIZATION 

We  find  experimentally  that  certain  solutions  will  peptize  or 
disintegrate  certain  precipitates  so  that  we  get  colloidal  solutions. 
This  is  always  the  result  of  adsorption.  Theoretically  there  are 
three  possibilities.  If  an  adsorbed  film  has  a  low  surface  tension 
on  the  water  side  and  a  high  one  on  the  other  side,  it  will  tend  to 
scrunch  up  and  to  peptize  the  solid  as  internal  phase.  If  the 
reverse  is  the  case,  the  solid  will  tend  to  form  the  external  phase. 
If  the  two  surface  tensions  are  practically  equal,  neither  will 
prevail.  Some  adsorbed  substances  seem  to  have  practically 
no  peptizing  action;4  but  these  cases  should  be  studied  again  in 
the  light  of  what  we  now  know  about  colloid  chemistry.  Tenta- 

1  WATSON:  Jour.  Chem.  Soc.,  60,875(1891);  GILBERT:  Jour.  Phys.  Chem., 
18,  86  (1914). 

2  MOURET:  Comptes  rendus,   123,  54   (1896);  DOELTER:   Zeit.   Kolloid- 
chemie,  7,  29,  86  (1910);  GALPIN:  Trans.  Am.  Ceramic  Soc.,  14,  337  (1912). 

3  GUTBIER:  Zeit.  Kolloidchemie,  4,  308  (1909). 

4  DOYLE:  Jour.  Phys.  Chem.,  17,  390  (1913). 


COALESCENCE  167 

tive  theories  of  peptization  have  been  discussed  by  Lottermoser, l 
Jordis,2  Mecklenburg,3  and  von  Weimarn.4  If  we  adopt  Freund- 
lich's  view  that  adsorption  always  lowers  the  surface  tension,  a 
theory  of  peptization  follows  at  once.  We  may  have  peptization 
by  a  liquid,  by  a  non-electrolyte,  by  an  adsorbed  ion,  by  a  salt,  or 
by  a  peptized  colloid.  When  a  liquid  is  adsorbed  by  a  solid, 
it  will  tend  to  peptize  it  and  in  some  cases  will  do  so.  Water 
peptizes  tannin,  amyl  acetate  peptizes  pyroxylin,  and  fused 
baths  peptize  metals.  At  higher  temperatures  the  peptizing 
action  increases,  and  we  may  get  glass  peptized  by  water5  or 
vulcanized  rubber  by  various  organic  liquids.6  If  the  amount 
of  substance  peptized  by  a  given  mass  of  water  under  given  con- 
ditions is  fairly  constant,  it  will  look  as  though  we  were  dealing 
with  true  solubility.  In  fact,  data  are  to  be  found  in  the  litera- 
ture for  the  solubility  of  tannin  in  water. 

There  are  a  number  of  cases  where  mixed  solvents  will  peptize 
a  solid  much  better  than  either  one  alone — cellulose  nitrates  in 
ether  and  alcohol,  caseine  in  pyridine  and  water,7  and  probably 
cinchonine  in  chloroform  and  alcohol,8  as  well  as  phloretine  in 
ether  and  water.9  The  theory  of  this  has  not  been  worked  out. 
Cellulose  nitrate  swells  in  alcohol  and  not  in  ether;10  but  it  is  not 
known  whether  this  is  universal.  We  do  not  know  whether 
alcohol  peptizes  cellulose  nitrate  at  higher  temperatures.  Zein 
is  also  peptized  in  mixed  solvents.11  Larguier  des  Bancels12  claims 
that  gelatine  is  peptized  more  readily  by  aqueous  alcohol  or 
aqueous  acetone  than  by  water  alone. 

1  Jour,  prakt.  Chem.  (2)  68,  341  (1903);  72,  39  (1905);  73,  374  (1906); 
Zeit.  phys.  Chem.,  62,  371  (1908). 

2  Van  Bemmelen  Gedenkboek,  215  (1910). 

3  Zeit.  anorg.  Chem.,  74,  260  (1912). 

4  Lehre  von  den  Zustanden  der  Materie,  1,  160  (1914). 

5BABUs:  Am.  Jour.  Sci.  (3)  38,  408  (1869);  41,  110  (1891);  (4)  6,  270 
(1898);  7,  1  (1899);  Phil.  Mag.  (5)  47,  104,  461  (1899). 
6  BARUS:  Am.  Jour.  Sci.  (3)  42,  359  (1891). 
7L,EViTEs:  Zeit.  Kolloidchemie,  8,  4  (1911). 

8  OUDEMANNS:  Jour.  Chem.  Soc.,  26,  533  (1873). 

9  SCHIFF:  Zeit.  phys.  Chem.,  23,  355  (1897). 

10  Private  communication  from  Professor  Chamot. 

11  GALEOTTI  and  GIAMPALMO:  Zeit.  Kolloidchemie,  3,  118  (1908). 
>2  Comptes  rendus,  146,  290  (1911). 


168  APPLIED  COLLOID  CHEMISTRY 

Relatively  little  work  has  been  done  on  direct  peptization  by 
means  of  a  dissolved  non-electrolyte,  but  a  good  deal  of  stress 
has  been  laid  on  the  cases  where  a  non-electrolyte  prevents  the 
formation  of  a  visible  precipitate.  A  concentrated  solution  of 
sugar  in  water  will  prevent  the  precipitation  of  calcium  silicate,1 
silver  chromate,  and  silver  chloride;2  also  of  lime  and  of  the 
hydrous  oxides  of  copper3  uranium,  and  iron.4  Invert  sugar  is 
about  seven  times  as  effective  as  cane  sugar  in  holding  up  hydrous 
ferric  oxide.  Grimaux5  showed  that  glycerine  prevents  the  pre- 
cipitation of  hydrous  ferric  oxide  by  caustic  potash. 

If  one  ion  of  an  electrolyte  is  adsorbed  more  than  the  other  ion, 
it  will  tend  to  peptize  the  adsorbing  material  and  to  give  rise  to  a 
colloidal  solution  containing  positively  or  negatively  charged 
particles  according  to  the  nature  of  the  adsorbed  ion.  Freshly 
precipitated  silver  halides  are  peptized  by  dilute  silver  nitrate 
or  the  corresponding  potassium  halide,6  the  silver  and  the  halide 
ions  being  adsorbed  strongly.  Many  oxides  are  peptized  by 
their  chlorides  or  nitrates,  forming  so-called  basic  salts.7  Sul- 
phides are  peptized  by  hydrogen  sulphide.8  Gelatine  is  liquefied 
or  peptized  by  a  potassium  iodide  solution.  The  peptization 
of  hydrous  oxides  by  caustic  alkali  can  be  considered  as  a  case  of 
common  ion  or  as  the  preferential  adsorption  of  hydroxyl  ion. 
Hydrous  chromic  oxide  gives  an  apparently  clear  green  solution 
when  treated  with  an  excess  of  caustic  potash;  but  the  green 
oxide  can  be  filtered  out  completely  by  means  of  a  collodion 

1  WEISBERG:  Bull.  Soc.  chim.,  Paris,  (3)  16,  1097  (1896). 

2  LOBBY  DE  BRUYN:  Ber.  deutsch.  chem.  Ges.,  35,  3079  (1902). 

3  GRAHAM:  Jour.  Chem.  Soc.,  15,  253  (1862). 

4  RIFFARD:  Comptes  rendus,  77,  1103  (1873). 

6  Comptes  rendus,  98,  1485,  1540  (1884). 

6LoTTERMosER:  Jour,  prakt.  Chem.  (2)  60,  341 '(1903);  72,  39  (1905);  73, 
374  (1906);  Zeit.  phys.  Chem.,  62,  371  (1908). 

7  GRAHAM:  Jour.  Chem.  Soc.,  15,  254  (1862);   MULLER:  Ber.  deutsch. 
chem.  Ges.,  39,  2856  (1906);  Zeit.  anorg.  Chem.,  52,  316  (1907);  SZILARD: 
Jour.  Chim.  phys.,  5,  488,  636  (1907). 

8  SPRING:  Ber.  deutsch.  chem.  Ges.,  16,  1142  (1883);  PROST:  Jour.  Chem. 
Soc.,   54,  653   (1888);  WINSSINGER:  Bull.   Soc.   chim.,   Paris   (3)   49,  452 
(1888);  LINDER  and  PICTON:  Jour.  Chem.  Soc.,  61,  116  (1892);  MEUNIER: 
Comptes  rendus,  124,  1151  (1897);  YOUNG:  Jour.   Phys.   Chem.,   21,  141 
(1917). 


COALESCENCE  169 

filter,  a  colorless  solution  passing  through.1  Hantzsch2  considers 
that  hydrous  beryllium  oxide  is  peptized  by  caustic  alkali3  and 
so  is  cobalt  oxide.4  In  ammoniacal  copper  solutions  part  of  the 
copper  oxide  it  apparently  colloidal  and  part  dissolved.5  Freshly 
precipitated  zinc  hydroxide  is  peptized  by  alkali;  but  the  solution 
is  very  unstable,  the  zinc  hydroxide  often  coagulating  inside 
half  an  hour.  The  relatively  small  amount  of  zinc  remaining 
in  solution  is  present  chiefly  or  entirely  as  sodium  zincate.6 
The  bulk  of  the  evidence  seems  to  be  that  alumina  is  not  peptized 
appreciably  by  alkali  and  that  it  goes  into  solution  as  sodium 
aluminate, 7  though  the  other  view  has  been  supported. 8  Adsorp- 
tion of  hydroxyl  ion  accounts  for  the  peptization  of  silicic  acid9  and 
of  caseine  by  alkalies.  Caseine  can  also  be  peptized  by  acids. 
A.  Miiller10  has  prepared  colloidal  solutions  of  aluminum,  iron, 
cobalt,  thorium,  and  yttrium  oxides  by  peptization  with 
dilute  hydrochloric  acid,  and  Bentley  and  Rose11  have  peptized 
freshly  precipitated  alumina  with  8  percent  acetic  acid.  It  is 
possible  but  not  probable,  that  the  peptization  is  done  by  a  trace 
of  metallic  salt  formed  by  the  acid  and  not  by  the  hydrogen  ion. 
There  are  no  cases  where  it  has  been  shown  conclusively  that 

1  FISCHER  and  HERZ:  Zeit.  anorg.  Chem.,  31,  352  (1902);  FISCHER:  Ibid., 
40,  39  (1904);  NAGEL:  Jour.  Phys.  Chem.,  19,  331,  569  (1915). 

2  Zeit.  anorg.  Chem.,  30,  289  (1902). 

3LoEw:  Zeit.  anal.  Chem.,  8,  463  (1870);  FISCHER:  Zeit.  anorg.  Chem., 
40,  39  (1904). 

*TTJBANDT:  Zeit.  anorg.  Chem.,  46,  368  (1905). 

6  PELIGOT:  Ann.  Chim.  Phys.  (3)  63,  343  (1861);  GUIGNET:  Comptes 
rendus,  66,  741  (1862);  GRIMAUX:  Ibid.,  89,  1434  (1884). 

6  HANTZSCH:  Zeit.  anorg.  Chem.,  30,  289  (1902);  76,  371  (1912);  FISCHER 
and  HERZ:  Zeit.  anorg.  Chem.,  31,  352  (1902);  KLEIN:  Zeit.  anorg.  Chem. 
74,  157  (1912). 

7  HERZ:  Zeit.   anorg.    Chem.,   26,   155    (1900);  HANTZSCH:  Zeit.  anorg. 
Chem.,  30,  289  (1902);  RUBENBAUER:  Zeit.  anorg.  Chem.,  30,  331  (1902); 
SLADE:  Jour.  Chem.  Soc.,  93,  421  (1908);  Zeit.  anorg.  Chem.,  77,  457  (1912); 
Trans.  Faraday  Soc.,  10,  150  (1914);  BLUM:  Jour.  Am.  Chem.  Soc.,  35,  1499 
(1913). 

8  MAHIN,  INGRAHAM  AND  STEWART:  Jour.  Am.  Chem.  Soc.,  36,  30  (1913). 

9  GRAHAM:  Jour.  Chem.  Soc.,  17,  324  (.1864). 

IOSVEDBERG:  Die  Methoden  zur  Herstellung  kolloider  Losungen  anor- 
ganischer  Stoffe,  400  (1909). 

11  Jour.  Am.  Chem.  Soc.,  35,  1490  (1913). 


170  APPLIED  COLLOID  CHEMISTRY 

peptization  is  due  chiefly  to  adsorption  of  undissociated  salt,1 
but  undoubtedly  such  instances  will  be  found.  Water-peptizable 
colloids  like  gelatine,2  gum  arabic,3  dextrine,4  soap,5  or  saponin6 
will  peptize  many  precipitates  and  they  are  often  called  protect- 
ing colloids  because  they  prevent  the  agglomeration  and  conse- 
quent settling  of  finely  divided  precipitates.  Caseine  is  not 
peptized  by  water;  but  acts  as  a  protecting  colloid  when  peptized 
by  acids  or  alkalies.  Hydrous  chromic  oxide  when  peptized  by 
caustic  potash,  can  then  prevent  the  precipitation  of  hydrous 
ferric  oxide,  etc.  If  too  much  ferric  oxide  is  present,  all  the 
chromic  oxide  is  carried  down  by  it.7  Solutions  of  copper  oxide 
in  ammonia  will  peptize  chromic  oxide.8  Molybdic  acid  is  not 
precipitated  from  its  salts  by  uranyl  salts,  but  tungstic  acid  is. 
In  presence  of  tungstic  acid,  practically  all  the  molybdic  acid  is 
precipitated.  This  is  obviously  a  case  of  adsorption  and  the 
converse  is  also  true  that  no  tungstic  acid  would  be  precipitated 
in  presence  of  a  sufficient  excess  of  a  molybdate.9  Aniline  dyes, 
which  are  insoluble  in  benzene  can  be  peptized  by  a  benzene- 
soluble  colloid  such  as  the  so-called  zinc  or  magnesium  resinate.10 
Since  a  colloidal  solution  is  one  in  which  a  finely  divided  phase 
is  kept  from  coalescing  in  some  way,  it  is  clear  that  there  may  be 
any  number  of  colloidal  aluminas,  for  instance,  varying  from 
anhydrous  alumina  (A12O3)  up  to  the  most  highly  hydrous  form 
that  can  be  obtained.  As  a  matter  of  fact,  people  have  generally 
been  satisfied  with  distinguishing  only  two  sets  of  colloidal  solu- 
tions, which  they  have  called  solutions  of  alumina  and  metal- 

1  It  seems  probable  that  the  peptization  of  caseine  by  acetic  acid  is  due 
largely  to  the  undissociated  acid. 

2Eder's  Handbuch  der  Photographic,  5th  Ed.,  3  I,  28  (1902);  LUPPO- 
CRAMER:  Phot.  Correspondenz,  44,  578  (1907). 

3LEFORT  and  THIBAULT:  Jour.  Chem.  Soc.,  42,  1322  (1882). 

4LACHAUDI  Bull.  Soc.  chim.,  Paris  (3)  16,  1105  (1896). 

6  SPRING:  Zeit.  Kolloidchemie,  4,   161   (1909);  6,  11,   109,   164  (1910). 
6ScHiAPARELLi:  Jour.  Chem.  Soc.,  46,  333  (1884). 

7  NORTHCOTE  and   CHURCH:  Jour.   Chem.   Soc.,   6,  54  (1854);   NAGEL: 
Jour.  Phys.  Chem.,  19,  331  (1915). 

8  PRUDHOMME:  Jour.  Chem.  Soc.,  25,  672  (1872). 

9  Miss  HITCHCOCK:  Jour.  Am.  Chem.  Soc.,  17,  483,  520  (1895);  WOHLER: 
Zeit.  Elektrochemie,  16,  693  (1910);  WOHLER  and  ENGELS:  Kolloidchem- 
ische  Beihefte,  1,  454  (1910). 

10  SOXHLET:  Art  of  Dyeing  and  Staining  Marble,  etc.,  76  (1902). 


COALESCENCE  171 

lumina,  of  stannic  and  metastannic  acids,  etc.1  While  adsorption 
will  cause  peptization  under  suitable  conditions,  the  disintegrat- 
ing power  of  the  adsorbed  substance  is  relatively  small  and  often 
will  not  be  sufficient  to  break  up  solid  masses.  A  protecting 
colloid,  for  instance,  will  prevent  the  formation  of  a  precipitate 
when  it  may  not  be  able  to  disintegrate  a  massive  precipitate. 

1  HANTZSCH:  Zeit.  anorg.  Ghem.,  30,  338  (1902). 


CHAPTER  VI 
PREPARATION  OF  COLLOIDAL  SOLUTIONS 

The  methods  of  making  colloidal  solutions  can  be  grouped 
under  two  heads,  the  condensation  methods  and  the  dispersion 
methods.  The  condensation  methods  will  be  considered  first. 

CONDENSATION  METHODS 

In  condensation  methods  we  must  have  strong  adsorption 
(including  protecting  colloids),  or  we  must  keep  down  the  con- 
centration of  agglomerating  agents  either  by  choice  of  reagents 
or  by  dilution.  A  few  instances  will  be  given  of  the  preparation 
of  colloidal  solutions  in  the  presence  of  a  strongly  adsorbed  sub- 
stance. Colloidal  palladium  may  be  obtained  by  reduction 
with  hydrazine  in  presence  of  gum  arabic.1  Colloidal  solutions 
of  silver  chloride,  silver  bromide,  or  Prussian  blue  may  be  ob- 
tained by  precipitation  in  presence  of  gelatine.2  Sulphide  solu- 
tions may  be  stabilized  by  gum  arabic3  or  by  sodium  sulphide, 
but  not  by  ammonium  sulphide.4  Silver  chromate,  hydrous 
copper  oxide,5  and  hydrous  ferric  oxide  may  be  obtained  colloidal 
in  presence  of  sugar.6  Invert  sugar  holds  up  about  seven  times 
as  much  iron  as  cane  sugar.  Glycerol  also  enables  us  to  make  a 
colloidal  solution  of  ferric  oxide.7  Gengou8  prepared  colloidal 
alumina  and  barium  sulphate  in  presence  of  sodium  citrate,  and 
Schiaparelli9  held  up  lead  sulphide  with  saponin.  A  solution 

1  SVEDBERG  :  Die  Methoden  zur  Herstellung  kolloider  Losungen  anorgan- 
ischer  Stoffe,  105  (1909). 

2  SVEDBERG,  323. 

3LEFORT  and  THIBAULT:  Jour.  Chem.  Soc.,  42,  1322  (1882). 

4  HAUSSMANN:  Zeit.  anorg.  Chem.,  40,  121  (1904). 

5  SVEDBERG,  224,  304. 

6RiFFARo:  Jour.  Chem.  Soc.,  27,  292  (1874). 

7  GRIMAUX:  Jour.  Chem.  Soc.,  46,  96  (1884). 

8  Zeit.  Kolloidchemie,  9,  88  (1911). 

9  Jour.  Chem.  Soc.,  46,  332  (1884). 

172 


PREPARATION  OF  COLLOIDAL  SOLUTIONS  173 

obtained  by  adding  sulphuric  acid  to  barium  arabinate  was  milky 
after  standing  four  years.  l  Cadmium  sulphide  can  be  obtained 
colloidal  in  presence  of  caseine,2  and  so  can  copper  oxide,3  while 
mercurous  chloride  can  be  held  up  by  albumin.4  All  the  metals 
and  many  other  substances  can  be  kept  in  colloidal  solution  by 
sodium  protalbinate  and  lysalbinate,  which  are  salts  of  somewhat 
hypothetical  acids  obtained  by  saponification  of  albumin.5  It 
is  an  open  question  whether  they  are  any  more  effective  than 
gelatine.  Colloidal  solutions  of  silver  in  organic  solvents  can 
be  obtained  with  wool-fat  as  protecting  colloid.6  Carey  Lea 
prepared  colloidal  silver  of  various  colors  by  reduction  of  a  silver 
salt  with  ferrous  citrate  or  tartrate.7  Both  red  and  blue  copper 
solutions  have  been  obtained  in  presence  of  lysalbinates.8  Col- 
loidal gold  and  silver  can  be  kept  from  coagulating  by  means  of 
starch.9  Mercury  and  bismuth  may  be  kept  up  by  stannic  oxide10 
and  gold  by  ceria,11  while  sodium  sulphate  keeps  up  sulphur  in 
strongly  acid  solution  and  potassium  sulphate  precipitates  it.12 
Acheson  aquadag  is  kept  in  colloidal  solution  by  tannin  and  iron 
oxide.13 

In  a  number  of  other  cases  the  concentration  of  the  agglomerat- 
ing agent  is  kept  low  either  by  selection  of  the  reaction  or  by 
dilution.  Since  electrolytes  are  very  effective  in  producing  pre- 
cipitation in  many  cases,  this  usually  means  keeping  the  concen- 
tration of  electrolytes  very  low.  It  may  make  matters  clearer 
to  classify  the  methods  according  to  the  type  of  reaction  involved. 

1.  Reduction  without  Electrolysis. — At  50°  hydrogen  gas 
reacts  slowly  with  silver  oxide  in  presence  of  water,  the  dissolved 

1  Jour.  Chem.  Soc.,  30,  538  (1876). 

2  SVEDBERG,  325. 

'RITTENHAUSEN:  Jour,  prakt.  Chem.  (2)  5,  215  (1873);  7,  361  (1874). 
4  SVEDBERG,  325. 

6  SVEDBERG,  125,  295,  327. 

6AMBERGER:  Zeit.  Kolloidchemie,  11,  99  (1912). 

7  SVEDBERG,  211. 

8  SVEDBERG,  166,  170. 

9  GUTBIER  and  WEINGARTNER:  Kolloidchemische  Beihefte,  6,  211,  244 
(1913). 

10  SVEDBERG,  204. 

11  SCHOTTLANDER:  Jour.  Chem.  Soc.,  68  II,  218  (1895). 

12  SVEDBERG,  249;  Cf.  ODEN:  Zeit.  Kolloidchemie,  8,  186  (1911). 
"  Cf.  DOYLE:  Jour.  Phys.  Chem.,  17,  390  (1913). 


174  APPLIED  COLLOID  CHEMISTRY 

silver  oxide  being  the  substance  which  reacts.  The  reaction  is 
Ag2O  +  H2  =  2Ag  +  H2O,  so  the  silver  is  formed  in  the  absence 
of  electrolytes  and  tends  to  come  down  colloidal,  though  some 
does  precipitate  as  a  mirror.1  From  dilute  solutions  of  gold  salts, 
colloidal  gold  can  be  obtained  by  reduction  with  carbon  monoxide, 
sulphur  dioxide,  formaldehyde,  pinene,  alcohol,  hydrazine,  and 
phenyl  hydrazine.2  In  these  cases  the  concentration  of  electro- 
lytes is  kept  low  by  dilution.  On  the  other  hand,  colloidal  cop- 
per can  be  obtained  by  the  action  of  alcohol  on  anhydrous  copper 
sulphate  and  concentrated  sulphuric  acid,3  a  reaction  which 
cannot  be  classified  without  further  study.  Colloidal  gold  can 
also  be  obtained  by  reduction  by  means  of  Aspergillus  oryzae.* 
The  effect  of  concentration  has  been  studied  by  Liversidge.5 
Colloidal  metals  may  also  be  obtained  by  reduction  in  flames.6 

2.  Reduction  with  Electrolysis. — When  very  dilute  solutions  of 
gold,  silver,  and  mercury  salts  are  electrolyzed,  some  of  the  metal 
comes  out  in  colloidal  form.7    Billitzer8  obtained  colloidal  solu- 
tions in  a  similar  way  with  salts  of  the  less  noble  metals,  but  it  is 
probable  that  the  dispersed  phase  consisted  of  oxide  and  not  of 
metal.9    Lecoq10  has  prepared  colloidal  arsenic  electrolytically, 
and  Sansonoff11  colloidal  UC>2,the  latter  being  stabilized  byuranyl 
chloride. 

3.  Oxidation. — Colloidal  sulphur  can  be  obtained  from  the 
reaction12  between  H2S  and  SO2,  which  involves  no  high  concentra- 
tion of  electrolyte.     In  presence  of   gelatine   as   stabilizer,  col- 
loidal sulphur13  can  be  obtained  by  oxidation  of  H2S.     Wohler 
and  Witzmann14  have  prepared  iridium  dioxide  as  a  violet  or 

1  SVEDBERG,  38. 

2  SVEDBERG,  60-71,  101,  112. 

3  RASENFOSSE:  Jour.  Soc.  Chem.  Ind.,  30,  133  (1911). 

4  SVEDBERG,  199. 

5  Jour.  Chem.  Soc.,  60,  401  (1891). 

6  DONAU:  Zeit.  Kolloidchemie,  16,  81  (1915). 

7  SVEDBERG,  206. 

8Ber.  deutsch.  chem.  Ges.,  35,  1929  (1902). 
9  HABER:  Zeit.  Elektrochemie,  8,  551  (1902). 

10  Comptes  rendus,  160,  700  (1910). 

11  Zeit.  Kolloidchemie,  8,  96  (1911). 

12  SVEDBERG,  243. 

13  HIMMELBAUR:  Zeit.  Kolloidchemie,  4,  307  (1909). 

14  Zeit.  anorg.  Chem.,  57,  328  (1908). 


PREPARATION  OF  COLLOIDAL  SOLUTIONS  175 

blue  solution.  Bredig  and  Marek1  have  obtained  colloidal 
manganese  dioxide  by  the  action  of  hydrogen  peroxide  on  per- 
manganate. When  aurous  chloride  breaks  down  into  auric 
chloride  and  gold,  the  latter  can  be  obtained  colloidal  under  suit- 
able conditions.2 

4.  Hydrolysis. — Water  will  hydrolyze  any  salt  until  the  pro- 
duct of  concentrations  of  the  hydroxyl  and  hydrogen  ions  reaches 
a  value  of  about  10~14.     If  either  base  or  acid  is  very  sparingly 
soluble,  the  hydrolysis  will  run  farther  than  if  both  are  strong 
electrolytes.     Equilibrium  will  be  reached  more  rapidly  if  the 
solution  is  heated.     If  a  dilute  solution  of  ferric   chloride  is 
boiled  for  a  few  minutes  and  then  cooled,  it  will  not  be  blackened 
by  hydrogen  sulphide3  and  it  will  not  form  Prussian  blue  with 
potassium  ferrocyanide.4     On  the  other  hand,  addition  of  sodium 
sulphate  or  of  sulphuric  acid  will  cause  a  precipitation  of  hydrous 
ferric  oxide.     Colloidal  solutions  of  the  hydrous  oxide  can  be 
obtained  by  hydrolysis  of  the  acetates,  the  nitrates,  or  the  chlo- 
rides, but  not  in  general  by  the  hydrolysis  of  sulphates,  because 
sulphuric  acid  precipitates  the  colloidal  solutions  more  readily 
than  hydrochloric,  nitric,  or  acetic  acid.     The  so-called  chrom- 
sulphuric  acids  are  apparently  special  cases  of  colloidal  solutions 
due  to  hydrolysis  in  sulphate  solutions.5     Colloidal  solutions 
can  also  be  obtained  by  hydrolysis  of  a  few  compounds  which 
are  not  electrolytes,6  such  as  silicon  sulphide,  ferric  ethylate, 
and  copper  succinimide. 

5.  Metathetical   Reactions. — When   hydrogen   sulphide   acts 
on  arsenious  oxide,  arsenious  sulphide  is  formed  practically  in 
the  absence  of  electrolytes,  As203  +  3H2S  =  As2S8  +  3H2O. 
This  colloidal  solution  can  be  made  of  the  very  high  concen- 
tration of  3  grams  As2S3  per  5  grams  water.7    The  reaction  be- 
tween mercuric  cyanide  and  hydrogen  sulphide  gives  prussic 
acid  in  solution;  but  this  is  a  very  weak  acid  and  has  but  slight 

1  Van  Bemmelen  Gedenkboek,  342  (1910). 

2  VANINO  and  ROESSLER:  Zeit.  Kolloidchemie,  6,  289  (1910). 

3  WRIGHT:  Jour.  Chem.  Soc.,  43,  156  (1883). 

4  SVEDBERG,  260. 

6  DENHAM:  Zeit.  anorg.  Chem., "67,  378  (1908);  STRONG:  Comptes  rendus, 
160,  1172  (1910). 

c  SVEDBERG,  276. 

7  SVEDBERG,  297. 


176  APPLIED  COLLOID  CHEMISTRY 

precipitating  power.1  Colloidal  sulphides  can  also  be  obtained 
with  salts  of  strong  acids  by  working  in  sufficiently  dilute  solu- 
tions.2 In  some  cases  sodium  chloride  has  relatively  little  pre- 
cipitating power,  and  it  is  therefore  possible  to  prepare  colloidal 
solutions  of  silicic  acid  from,  sodium  silicate,  of  the  so-called 
stannic  and  metastannic  acids  from  stannic  chloride  and  sodium 
stannate  respectively;3  and  of  tungstic  acid  from  sodium  tung- 
state.  The  tungstic  acid  solution  can  be  concentrated  until  it 
contains  80  percent  tungstic  acid  and  has  a  density  of  over  3.2, 
so  that  glass  will  float  in  it.4 

If  M/20  AgN03  is  mixed  with  M/20  KC1,  KBr,  or  KI,  so  that 
there  is  a  slight  excess  of  one  or  the  other  solution,5  more  or  less 
of  the  silver  halide  stays  in  suspension,  being  charged  positively 
in  the  one  case  owing  to  Adsorption  of  silver  ion,  and  charged 
negatively  in  the  other  case  owing  to  adsorption  of  halide  ion. 
A  salt  which  is  readily  soluble  in  water  can  be  obtained  in  a 
colloidal  state  by  working  in  a  solvent  in  which  the  salt  is  practi- 
cally insoluble  and  therefore  precipitates  in  an  extremely  finely 
divided  state.6  Thus  colloidal  sodium  chloride  can  be  obtained 
by  the  interaction  of  sodium  malonic  ester  and  chloracetic  ester 
in  ligroin.7 

6.  Decomposition  by  Light. — Colloidal  solutions  can  be  ob- 
tained when  substances  are  precipitated  by  the  action  of  light. 
Svedberg8  has  studied  the  action  of  light  on  gold  solutions,  and 
Wigand9  the  behavior  of  sulphur  solutions.  There  are  also 
several  other  interesting  papers  along  similar  lines.10 

1  SVEDBERG,  302. 

2  WINSSINGER:  Jour.  Chem.  Soc.,  54,  911  (1888). 

3  SVEDBERG,  303. 

«  MULLFR:  Van  Bemmelen  Gedenkboek,  416  (1910). 

6LoTTERMOSER:  Jour,  prakt.  Chem.  (2)  68,  341  (1903);  72,  39  (1905);  73, 
374  (1906);  cf.  DRUCKER:  Zeit.  Kolloidchemie,  4,  216  (1910). 

8  BOTTGER:  Jour.  Chem.  Soc.,  36,  107  (1878);  VON  WEIMARN:  Zeit. 
Kolloidchemie,  7,  92  (1910);  8,  216  (1911);  11,  651  (1912);  cf.  PIERONI: 
Gazz.  chim.  ital.,  43  I,  197;  PATERN&  and  MEGIGRECIANU,  Ibid.,  43  I,  439 
(1913). 

7  SVEDBERG,  349. 

8  Zeit.  Kolloidchemie,  6,  238  (1910). 

9  Zeit.  phys.  Chem.,  77,  423  (1911). 

IOAMANN:  Zeit.  Kolloidchemie,  8,  197  (1911);  GALECKI:  Ibid.,  10,  149 
(1912);  HARTWAGNER:  Ibid.,  16,  70  (1915). 


PREPARATION  OF  COLLOIDAL  SOLUTIONS  177 

DISPERSION  METHODS 

Five  different  types  of  preparation  of  colloidal  solutions  by 
peptization  may  be  distinguished: 

1.  Removal  of  Agglomerating  Agent. — If  a  precipitate  has 
settled  from  a  colloidal  solution  owing  to  the  addition  of  too  much 
of  an  agglomerating  agent,  the  precipitate  may  go  back  into 
apparent  solution  if  the  excess  of  agglomerating  agent  is  washed 
out.     No  colloidal  solution  will  be  obtained  if  it  is  impossible  to 
wash  out  the  coagulating  agent  or  if  the  agglomeration  has  gone 
too  far.1     If  a  silver  halide  precipitate  is  washed  on  a  filter  at 
once,  the  silver  salt  is  apt  to  run  through  the  filter  when  the 
excess  of  potassium  salt  has  been  removed  and  there  is  present 
only  the  amount  which  would  have  kept  the  silver  halide  in  sus- 
pension originally.     When  the  rare  earth  nitrates  are  precipitated 
by  ammonia  and  then  washed,  they  are  liable  to  stay  suspended 
in  the  liquid  when  the  ammonium  nitrate  is  nearly  all  removed. 
In  alloy  work  stannic  oxide  should  be  washed  with  dilute  nitric 
acid  and  not  with  water.     Zinc  sulphide  is  apt  to  form  a  colloidal 
solution  when  the  ammonium  salt  is  washed  out,2  and  copper 
ferrocyanide  does  the  same  thing  if  all  the  copper  sulphate  is 
removed    by    washing.3     In    fact,    Chatard4    claims    that    the 
quickest  way  to  wash  a  gelatinous  precipitate  is  to  evaporate  the 
solution  to  dryness  and  heat  before  trying  to  wash.     Merely 
evaporating  on  a  water-bath  is  not  always  sufficient.5 

2.  Addition  of  Peptizing  Agent. — Instead  of  washing  out  a  pre- 
cipitating agent,  a  peptizing  agent  may  be  added.     Ammonia  is 
very  effective  in  suspending  clay,6  silicic  acid  is  peptized  readily 
by  caustic  soda,7  and  Prussian  blue  is  peptized  by  oxalic  acid  or 
by  potassium  oxalate.     In  some  cases  the  concentration  of  the 
peptizing  agent  has  to  be  high,  as  when  oxides  are  to  be  peptized 
by  alkali,  and  people  usually  assume  the  formation  of  compounds. 

1  Cf.  ABEGG  and  VON  SCHROEDER:  Zeit.  Kolloidchemie,  2,  85  (1907). 

2  DONNINI:  Jour.  Chem.  Soc.,  66  II,  318  (1894). 

3  BERKELEY  and  HARTLEY:  Phil.  Trans.,  206A,  486  (1906). 
*  Jour.  Chem.  Soc.,  26,  527  (1873). 

6  WRIGHT:  Jour.  Chem.,  43,  156  (1883);  KRATZ:  Jour.  Phys.  Chem.,  16, 
121  (1912). 

6SKEY:  Chem.  News,  17,  164  (1868);  22,  236  (1870);  34,  142  (1876). 
See  also  DOELTER:  Handbuch  der  Mineralchemie,  2,  122  (1912). 

7  GRAHAM:  Jour.  Chem.  Soc.,  17,  324  (1864). 

12 


178  APPLIED  COLLOID  CHEMISTRY 

Jowitschitsch1  describes  conditions  under  which  chromic  oxide 
is  apparently  completely  soluble  in  ammonia.  This  is  probably 
a  case  of  peptization. 

The  action  of  soap  on  rouge  or  carbon  black2  looks  like  a  dis- 
integration; but  it  is  not.  If  a  suspension  of  carbon  black  in 
water  be  filtered  several  times  through  filter-paper,  the  water 
will  finally  run  through  clear,  and  the  carbon  black  will  be  held 
back  by  the  filter-paper.  If  a  soap  solution  be  poured  on  the 
filter,  a  black  filtrate  is  obtained  and  the  filter-paper  is  no  longer 
black.  All  the  carbon  black  has  passed  through  the  filter-paper. 
The  same  thing  can  be  done  with  rouge,  except  that  a  red  filtrate 
is  obtained  instead  of  a  black  one.  At  first  sight  it  seems  as 
though  the  soap  must  have  broken  up  the  carbon  or  the  rouge 
into  finer  particles,  which  then  passed  through  the  filter,  but  this 
is  probably  not  so.  The  filter-paper  is  porous  enough  at  first  to  let 
through  the  particles  of  carbon  or  rouge,  as  is  shown  by  the  fact 
that  some  of  the  suspended  matter  does  pass  through  the  filter  at 
first.  The  cellulose  adsorbs  the  carbon  black  or  the  rouge,  and 
and  this  clogs  the  filter  to  such  an  extent  that  the  pores  are  not 
large  enough  to  let  the  remaining  particles  through.  The  soap 
removes  the  rouge  or  the  carbon  black  from  the  paper  because  it 
adsorbs  these  substances  more  strongly,  and  everything,  there- 
fore, goes  through  the  paper.  That  this  is  the  true  explanation 
can  be  shown  in  two  ways.  In  the  first  place,  the  experiment 
does  not  succeed  if  the  rouge  or  the  carbon  is  too  coarse.  In 
the  second  place,  Spring  showed  that  we  are  dealing  with  an 
adsorption  of  carbon  black  by  filter-paper.  If  the  black  filter- 
paper  be  reversed  and  washed  with  water,  the  water  removes 
only  the  black  which  is  not  in  immediate  contact  with  the  paper. 

3.  Mechanical  Disintegration. — If  a  solid  be  ground  up  suffi- 
ciently fine,  it  will  necessarily  form  a  colloidal  solution  for  a  time. 
This  has  been  done  experimentally  by  Wegelin3  in  the  case  of  a 
number  of  metals.  The  addition  of  gelatine  makes  it  easier  to 
disintegrate  ductile  metals.4  It  must  be  remembered  that  a 

1  Monatsheft  fur  Chemie,  34,  225  (1913). 

2  SPRING:  Zeit.  Kolloidchemie,  4,  161  (1909);  6,  11,  109,  164  (1909). 

3  Zeit.  Kolloidchemie,  14,  65  (1914). 

4  Metallic  inks  are  said  to  be  made  by  grinding  with  honey.     THORPE: 
Dictionary  of  Applied  Chemistry,  3,  138  (1918). 


PREPARATION  OF  COLLOIDAL  SOLUTIONS  179 

powder  made  with  gelatine  will  not  necessarily  have  exactly  the 
same  properties  as  the  same  powder  when  pure.  Tyndall1  has 
called  attention  to  one  classical  instance  of  an  error  due  to  over- 
looking the  effect  which  a  little  gum  might  have  on  the  adsorption 
of  dark  heat  rays.  Melloni  found  the  same  radiant  and  absorb- 
ent power  for  chalk  and  lamp-black.  Masson  and  Courte'pe'e 
studied  a  number  of  chemical  precipitates  and  found  that  all 
manifested  the  same  power  of  radiation.  The  conclusion  was 
drawn  that  the  influence  of  the  fine  state  of  division  is  so  power- 
ful as  entirely  to  mask  and  over-ride  whatever  influence  might  be 
due  to  chemical  constitution.  As  a  matter  of  fact,  the  precipi- 
tates were  mixed  with  a  solution  of  gum  in  water  and  were  then 
painted  on  the  side  of  a  cube  with  a  brush.  The  gum  coating  is 
very  transparent  to  light  but  as  opaque  as  pitch  or  lamp-black 
to  the  longer  non-luminous  rays.  The  measurements  all  re- 
ferred to  a  surface  of  gum  and  consequently  all  came  out 
alike. 

Since  a  finely  divided  solid  is  more  soluble,  it  is  possible  that  it 
may  go  into  solution  and  then  precipitate  in  another  form.  This 
seems  to  happen  with  quartz.  When  reduced  to  an  impalpable 
powder  by  long  grinding,  quartz  can  be  converted  into  colloidal 
hydrous  silicic  acid  merely  by  boiling  with  water.2 

4.  Electrical  Disintegration. — When  a  direct  current  arc  is 
formed  under  water  between  two  wires,  the  metal  is  disintegrated 
and  colloidal  solutions  of  platinum,  iridium,  palladium,  gold,  silver, 
and  cadmium  may  be  obtained  in  this  way.3  Satisfactory  condi- 
tions are  obtained  with  30  to  40  volts  and  5  to  10  amperes.  A 
trace  of  alkali  in  the  water  causes  formation  of  finer  particles, 
presumably  owing  to  the  stabilizing  effect  of  the  hydroxyl  ion. 
The  disintegration  is  chiefly  at  the  cathode. 

The  method  is  not  satisfactory  with  organic  liquids,  because 
too  much  decomposition  of  the  liquids  takes  place.  Svedberg 
found  empirically  that  this  decomposition  could  be  decreased 
very  much  if  the  current  density  were  made  as  small  as  possible.4 

1  Fragments  of  Science:  Radiant  Heat  and  its  Relations. 

2  DESCH:  The  Chemistry  and  Testing  of  Cement,  58  (1911). 

3  BREDIG:  Zeit.  Elektrochemie,  4,  514  (1898);  Zeit.  phys.  Chem.,  31,  258 
(1899). 

4  SVEDBERG,  424. 


180  APPLIED  COLLOID  CHEMISTRY 

He  therefore  used  an  oscillatory  discharge  from  an  induction 
coil  with  a  condenser  in  parallel  or  in  series.  The  best  results 
are  obtained  with  large  capacity,  small  self-induction,  low  resist- 
ance, and  short  arc.1  By  this  improved  method  Svedberg  suc- 
ceeded in  preparing  colloidal  solutions  of  all  the  metals,  including 
the  alkali  metals.  Liquid  methane,  ether,  and  isobutyl  alcohol 
at  low  temperature  were  especially  satisfactory  with  the  metals 
of  the  alkalies  and  the  alkaline  earths.  The  order  of  disintegra- 
tion of  some  of  the  metals  under  similar  conditions  was  found  to 
be  Fe,  Cu,  Ag,  Al,  Ca,  Pt,  Au,  Zn,  Sn,  Cd,  Sb,  Tl,  Bi,  Pb,  the  iron 
being  the  least  rapidly  disintegrated  and  the  lead  the  most  rapidly. 
There  is  no  apparent  relation  either  with  the  order  of  the  boiling- 
points  or  with  the  order  of  disintegration  by  cathode  rays  or 
canal  rays. 

5.  Electrochemical  Disintegration. — With  a  lead  cathode  in 
caustic  soda  solution,  the  lead  disintegrates  when  the  current 
density  exceeds  a  critical  value,  and  the  solution  is  colored  black 
like  ink  with  fine  particles  of  metallic  lead.2  This  is  due  to  the 
temporary  formation  of  a  sodium-lead  alloy,  which  then  disin- 
tegrates in  contact  with  water.  Similar  results  can  be  obtained 
with  cathodes  of  tin,  bismuth,  thallium,  arsenic,  antimony, 
and  mercury.  E.  Mtiller3  obtained  colloidal  solutions  of  tellur- 
ium with  a  tellurium  cathode.  This  seems  to  be  due  to  the 
formation  of  polytellurides,  which  break  down  and  set  free 
tellurium.  In  the  presence  of  oxygen  there  may  be  also  an  oxida- 
tion of  a  telluride.  Fischer4  has  obtained  metallic  copper  in  the 
solution  by  using  a  high  current  density  with  a  copper  anode  in 
sulphuric  acid.  Cuprous  sulphate  is  formed,  which  breaks  down 
to  metallic  copper  and  cupric  sulphate.  This  experiment  has 
not  yet  been  made  to  give  colloidal  copper,  but  this  could  prob- 
ably be  done  if  one  were  to  add  a  suitable  protecting  colloid. 

1  See  also  MORRI&-AIREY  and  LONG:  Jour.  Chem.  Soc.,  104  II,  1033  (1913); 
LONG:  Zeit.  Kolloidchemie,  14,  136  (1914);  KTTTSCHEROFF :   Ibid.,  11,  165 
(1912);  ZAWRIEFF:  Zeit.  phys.  Chem.,  87,  506  (1914). 

2  REED:  Jour.  Franklin  Inst.,  139,  283  (1895);  BREDIG  and  HABER:  Ber. 
deutsch.  chem.  Ges.,  31,  2741  (1898);  HABER  and  SACK:  Zeit.  Elektrochemie, 
8,  243  (1902);  Zeit  anorg.  Chem.,  34,  286  (1903). 

3  Zeit.  Elektrochemie,  11,  521,  701   (1905);  HABER:  Ibid.,  11,  660,  827 
(1905). 

4  Ibid.,  9,  507  (1903). 


PREPARATION  OF  COLLOIDAL  SOLUTIONS  181 

The  disintegration  of  all  electrodes  by  an  alternating  current 
when  the  current  density  is  high  is  undoubtedly  due  to  the  tem- 
porary formation  and  subsequent  breaking  down  of  a  hydrogen 
or  metallic  alloy.1 

1  HABER:  Trans.  Am.  Electrochem.  Soc.,  2,  192  (1902);  VAN  NAME:  Zeit. 
Elektrochemie,  10,  303  (1904). 


CHAPTER  VII 
PROPERTIES  OF  COLLOIDAL  SOLUTIONS 

GENERAL  PROPERTIES 

The  whole  theory  of  colloidal  solutions  is  based  on  the  assump- 
tion that  we  are  dealing  with  two-phase  systems.  Since  this  is 
not  obvious  and  is  still  disputed  by  some  people,  it  is  important 
to  see  what  independent  proof  we  have  of  the  existence  of  a 
second  phase.  If  a  beam  of  light  strikes  small  particles  in  sus- 
pension in  a  transparent  medium,  the  light  is  scattered  and  polar- 
ized, provided  the  refractive  index  of  the  particles  differs  from 
that  of  the  transparent  medium.  The  intensity  of  the  scattered 
light  is  inversely  proportional  to  the  fourth  power  of  the  wave- 
length and  the  scattering  by  each  particle  is  proportional  to  the 
square  of  the  volume  of  the  particle.1  The  scattered  light  is 
therefore  chiefly  blue.2  This  is  known  as  the  Tyndall  phenomenon 
and  it  was  used  by  him  to  determine  whether  air  was  free  from 
dust  and  was  optically  empty.  The  same  principle  has  been 
perfected  and  applied  in  the  ultra-microscope,3  so-called.  In 
this  instrument  a  powerful  beam  of  light  is  focussed  on  the 
liquid  to  be  examined  and  is  looked  at  from  the  side  through  a 
powerful  microscope.4  If  there  are  suspended  particles  in  the 
liquid,  they  appear  as  bright  points.  One  does  not  see  the 
particles  themselves  but  merely  the  light  diffracted  by  them. 
With  colloidal  gold  in  glass  it  is  possible  to  see  that  we  have  a 
two-phase  system.  By  means  of  some  form  of  this  instrument 
there  is  no  difficulty  in  demonstrating  the  existence  of  a  second 
phase  in  many  colloidal  solutions.  In  fact  the  instrument  is  so 
delicate  that  one  must  guard  against  the  danger  of  casual  im- 


i  Phil.  Mag.  (4),  41,  107,  447  (1871). 

2  Instead  of  calling  this  by  the  clumsy  term  of  the  blue  of  turbid  media, 
it  would  be  very  appropriate  to  speak  of  Tyndall  blue. 

3  SIEDENTOPF  and  ZSIGMONDY:  Drude's  Ann.,  10,  1  (1903). 

4  For  a  general  discussion  of  the  ultra-microscope,  see  BURTON:  The 
Physical  Properties  of  Colloidal  Solutions,  28  (1916). 

182 


PROPERTIES  OF  COLLOIDAL  SOLUTIONS  183 

purities  making  one  see  a  second  phase  when  it  is  not  there. 
This  difficulty  becomes  very  serious  with  liquids  other  than  water. 

The  recent  work  of  Martin1  seems  to  prove  that  some  organic 
liquids  scatter  light  even  when  absolutely  pure.  In  so  far  as 
this  may  be  true  for  a  pure  liquid  or  a  true  solution,  the  evidence 
of  the  ultra-microscope  is  not  conclusive  as  to  heterogeneity. 

Although  the  particles  themselves  are  not  seen,  it  is  possible  to 
estimate  their  size  if  one  knows  the  number  and  mass,  and  as- 
sumes that  the  particles  are  practically  spherical.  It  is  custom- 
ary to  use  the  word  " microns"  for  particles  with  diameters 
exceeding  250^,  while  those  particles  ranging  from  6-250/xju 
are  called  submicrons,  and  the  term  amicrons  is  used  for  particles 
with  a  diameter  less  than  6/^.  The  existence  of  these  smallest 
particles  can  be  shown  in  certain  cases  because  the  particles 
may  serve  as  nuclei  for  crystallization.  Kogonossoff2  claims  to 
have  followed  the  electrolysis  of  silver  nitrate  solutions  with  the 
ultra-microscope.  Since  the  visibility  of  particles  depends  on 
the  difference  in  index  of  refraction  of  the  particles  and  the  me- 
dium, it  is  evident  that  the  more  hydrous  the  second  phase,  the 
less  easily  can  it  be  seen.  With  many  organic  colloids  the 
particles  can  be  seen  only  when  they  exceed  40^  in  diameter. 
The  Brownian  movements  are  consequently  much  less  marked. 

It  is  possible  to  prepare  salt  solutions  which  are  optically 
empty.  On  the  other  hand  Lobry  de  Bruyn3  claims  to  have 
found  that  cane  sugar  solutions  were  optically  inhomogeneous. 
This  has  been  cited  as  a  case  of  a  large  molecule  being  visible; 
but  it  is  much  more  likely  that  the  scattering  was  due  to  an 
impurity  and  not  to  the  sugar  itself.  It  apparently  seems  obvi- 
ous to  most  people  that  if  one  continues  to  subdivide  a  precipi- 
tate sufficiently,  one  will  pass  from  a  suspension  to  a  true  solution 
without  a  break.4  The  fallacy  lies  in  the  assumption  that 
one  can  have  all  sizes  of  particles  in  equilibrium  with  the  liquid. 
Since  the  solid  becomes  more  soluble  the  more  finely  subdivided 
it  is,  there  will  be  particles  of  a  certain  size  which  will  dissolve 

1  Jour.  Phys.  Chem.,  24,  478  (1920). 

2  Zeit.  Kolloidchemie,  7,  129  (1910). 

3  Rev.  Trav.  chim.  Pays-Bas,  23,  218  (1904). 

4  See,  however,  VON  WEIMARN:  Zur  Lehre  von  den  Zustanden  der  Materie, 
161  (1914). 


184  APPLIED  COLLOID  CHEMISTRY 

and  reprecipitate  in  a  coarser  form  and  which  therefore  cannot 
coexist  for  any  length  of  time  with  the  liquid.  This  comes  out 
clearly  in  the  case  of  smokes,  it  being  very  difficult  to  obtain  a 
stable  smoke  of  satisfactory  concentration  with  particles  less 
than  lOju//.  If  one  takes  definite  amounts  of  benzene  and  water, 
it  is  possible  theoretically  to  subdivide  the  benzene  so  that  it 
will  dissolve  completely  in  the  water;  but  the  true  solution  thus 
formed  would  have  different  properties  from  the  corresponding 
suspension.  It  would  show  characteristic  osmotic  pressure 
phenomena  and  it  would  be  instable  in  quite  a  different  way  from 
the  suspension.  While  it  is  perfectly  possible  to  treat  a  solution 
formally  as  the  end  term  in  a  series  of  suspensions,  there  is 
nothing  as  yet  to  show  that  this  is  a  desirable  hypothesis.1 

The  presence  of  a  second  phase  in  a  colloidal  solution  can  also 
be  shown  by  what  is  called  ultra-filtration.  A  colloidal  solution 
goes  right  through  an  ordinary  filter;  but  Bechhold2  showed  that 
it  was  possible  to  form,  on  filter  paper  or  wire  gauze,  collodion 
films  with  varying  amounts  of  acetic  acid  or  gelatine  films  hard- 
ened with  formaldehyde,  such  that  pores  of  almost  any  desired 
size  can  be  obtained.  By  taking  a  filter  with  sufficiently  fine 
pores,  almost  any  colloidal  solution  can  be  filtered  so  as  to  leave 
the  suspended  matter  behind.  With  a  more  porous  filter  the 
finer  particles  go  through  and  the  coarser  ones  are  held  back. 
In  this  way  it  is  possible  to  get  an  approximate  idea  of  the  rela- 
tive sizes  of  the  particles  in  two  different  colloidal  solutions.  Solu- 
tions made  up  by  Bechhold  are  arranged  in  Table  XLII  in  the 
order  of  increasing  fineness  of  particles.  Blue  colloidal  solu- 
tions of  litmus  have  finer  particles  in  them  than  the  corresponding 
red  solutions. 

Solutions  made  up  in  other  ways  would  give  different  results, 
because  colloidal  gold  solutions,  for  instance,  can  be  made  with 
particles  of  very  different  sizes.  The  deep  green  solution  formed 
when  chromic  oxide  is  peptized  by  alkali  comes  through  color- 
less when  a  suitable  ultra-filter  is  used.  On  standing,  the  chro- 
mic oxide  eventually  becomes  so  coarse  that  it  can  be  separated 
by  means  of  an  ordinary  paper  filter.  In  all  cases  of  ultra-filtra- 

*Cf.  LEBLANC:  Zeit.  Elektrochemie,  19,  794  (1913). 
2  Zeit.  phys.  Chem.,  60,  257  (1907);  64,  328  (1908);  SCHOEP:  Zeit.  Kolloid- 
chemie,  8,  80  (1911);  MALFITAMO:  Comptes  rendus,  131,  1229  (1904). 


PROPERTIES  OF  COLLOIDAL  SOLUTIONS  185 

tion,  care  must  be  taken  to  ensure  that  a  real  nitration  is  taking 
place  and  that  the  results  are  not  vitiated  by  adsorption.  It  is 
to  be  noted  that  the  behavior  of  ultra-filters  can  be  accounted  for 
on  the  basis  of  size  of  pores  and  size  of  particles,  which  is  not 
true  for  semi-permeable  membranes.1  On  the  other  hand  there 
seems  to  be  some  difficulty  in  determining  the  absolute 
size  of  the  pores.  Experiments  by  the  bubble  method  with  the 
ultra-filter  indicated  that  the  size  of  the  colloidal  silver  (or  collar- 
gol)  particles  was  200^490juju  whereas  experiments  with  a  Cham- 
berland  filter  indicated  that  the  coarsest  collargol  particles  were 
less  than  170/i/x  in  diameter.  In  Table  XLII  they  are  called 

20/iM- 

TABLE  XLII. — ORDER  OF  ULTRAFILTRATION 


Coarse  suspensions 
Prussian  blue 
Platinum  sols  (Bredig) 
Iron  oxide 
Caseine 

Arsenious  sulphide 
Gold  (about  4(W) 
Bismuth  oxide 
Lysargin  (silver) 
Collargol  (silver,  20/i/i) 
Gold 


1  percent  gelatine 
1  percent  hemoglobin 
Serum  albumin 
Diphtheria  toxin 
Protalbumoaes 
Silicic  acid 
Lysalbinic  acid 
Deutero  albumoses 
Litmus 
Dextrin 
Crystalloids 


With  a  suitable  ultra-filter  it  is  possible  to  filter  ordinary  water 
so  that  it  is  absolutely  sterile.  It  is  also  possible  to  obtain  in 
this  way  water  which  can  be  used  for  work  with  the  ultra-micro- 
scope. The  behavior  of  mixtures  is  distinctly  interesting.  If 
the  two  substances  do  not  either  one  adsorb  the  other  and  if  the 
particles  are  of  different  sizes,  a  separation  can  be  effected.  The 
green  mixture  of  Prussian  blue  and  hemoglobin  filters  red  with  a 
suitable  filter,  the  fine  hemoglobin  going  through  and  the  coarse 
Prussian  blue  remaining  behind.  In  a  mixture  of  Prussian  blue 
with  arsenic  sulphide  or  ferric  oxide,  there  is  adsorption  and 
everything  is  stopped  by  a  filter  which  would  let  through  either 
the  arsenic  sulphide  alone  or  the  ferric  oxide  alone.  By  adding 
a  protecting  colloid,  sodium  lysalbinate,  to  Prussian  blue  further 
dispersion  takes  place  and  the  Prussian  blue  will  pass  through  a 
filter  which  just  stopped  it  before.  If  serum  albumin  is  added 

1  WALDEN:  Zeit.  phys.  Chem.,  10,  699  (1892). 


186  APPLIED  COLLOID  CHEMISTRY 

to  Prussian  blue  and  then  oxalic  acid,  the  Prussian  blue  particles 
are  found  to  be  smaller  than  before.  If  oxalic  acid  is  added  and 
then  the  serum  albumin,  no  marked  change  in  the  size  of  particles 
takes  place.  Undoubtedly  the  oxalic  acid  is  adsorbed  by  the 
Prussian  blue  and  prevents  the  adsorption  of  the  albumin  to  a 
considerable  extent. 

Since  the  existence  of  a  colloidal  solution  depends  on  the 
Brownian  movements  counteracting  the  force  of  gravity,  it 
follows  that  if  we  increase  the  force  of  gravity,  a  colloidal  solution 
will  become  less  stable  and  may  precipitate.  This  can  be  realized 
experimentally  by  centrif uging. l  The  best  known  case  of  this 
is  the  separator  in  which  the  cream,  being  lighter,  goes  to  the 
center  while  the  milk  is  thrown  to  the  outside.  A  few  precipita- 
tions of  other  solutions  have  been  made;  but  the  subject  has  not 
been  studied  systematically.  At  4000  revolutions  per  minute 
gold  submicrons  are  precipitated  in  a  few  minutes  while  the  fine 
amicrons  of  Fe2O3  are  not  precipitated.  With  a  super-centrifuge 
running  40,000  revolutions  per  minute  and  more,  it  is  possible  to 
precipitate  many  more  colloidal  solutions.  The  converse  is 
also  true,  that  a  solution  becomes  more  stable  against  centrifug- 
ing  if  the  particles  are  made  smaller.  A  separator  does  not  give 
cream  with  homogenized  milk  in  which  the  fat  particles  are  less 
than  0.8/-6  in  diameter.2  The  fact  that  a  true  solution,  if  practi- 
cally saturated,  can  be  made  to  crystallize  by  centrif  uging,3 
does  not  indicate  any  real  analogy  between  true  and  colloidal 
solutions.  In  the  case  of  the  true  solution,  only  a  small  amount 
of  the  dissolved  substance  can  be  precipitated  while  with  a 
colloidal  solution  practically  all  the  suspended  material  comes 
down.4 

A  coarse  suspension,  if  entirely  insoluble  and  anhydrous, 
should  give  rise  to  no  osmotic  pressure,  no  diffusion  except  as  a 
result  of  electrical  charges,  and  no  change  in  surface  tension  or 

1  FRANKLIN  and  FREUDENBERGER:  Trans.  Am.  Electrochem.  Soc.,  8,  29 
(1903);  GIOLITTI:  Gazz.   chim.  ital.,   36  II,   159   (1906);  DUMANSKI:  Van 
Bemmelen  Gedenkboek,  421;  FRIEDENTHAL:  Zeit.  Kolloidchemie,   15,    75 
(1914);  AYRES:  Met.  Chem.  Eng.,  14,  500  (1916). 

2  FLEISCHMANN:  Lehrbuch  der  Milch wirtschaft,  393  (1906). 

3  BREDIG:  Zeit.  phys.  Chem.,  17,  459  (1895);  LOBRY  DE  BRUYN  and  VAN 
CALCAR:  Rec.  Trav.  chim.  Pays-Bas,  23,  218  (1904). 

4  BECHHOLD:  Zeit.  Kolloidchemie,  2,  3  (1907). 


PROPERTIES  OF  COLLOIDAL  SOLUTIONS  187 

conductivity.  If  the  suspension  is  hydrous  it  may  have  a  marked 
effect  on  the  surface  tension.  In  fact  Lottermoser1  has  suggested 
that  the  hydrous  colloid  may  sometimes  be  the  external  phase. 
In  general  the  apparent  osmotic  pressures  of  colloidal  solutions 
are  very  small  and  this  has  led  to  absurd  molecular  weights.2 
Such  values  as  30,000  for  albumin,  700,000  for  glycogen,  over 
48,000  for  silica  and  "  enormous"  for  Fe203  mean  nothing  whatso- 
ever. This  can  be  seen  readily  if  we  consider  the  case  of  boiling 
water  to  which  scrap  platinum  is  added  to  prevent  overheating. 
Nobody  would  deduce  a  molecular  weight  of  platinum  from  such 
an  experiment,  not  even  if  the  platinum  were  sub-divided  a  good 
deal,  and  yet  this  is  just  what  people  do  continually  with  colloids. 
If  one  adds  water  to  benzene,  a  little  will  dissolve  and  will  change 
the  freezing  point  of  the  benzene.  From  this  one  can  calculate 
the  molecular  weight  of  water  dissolved  in  benzene;  but  the  calcu- 
lation means  nothing  so  soon  as  there  is  an  excess  of  water.  It 
would  still  mean  nothing  if  one  were  to  break  the  water  up  into 
drops  which  stayed  suspended  for  a  while  in  the  benzene. 

It  is  not  denied  that  colloidal  solutions  may  show  osmotic 
phenomena.  This  may  happen  in  case  the  suspended  material 
is  somewhat  soluble  or  contains  adsorbed  soluble  material.3  It 
is  wrong,  however,  to  refer  the  observed  osmotic  pressure  to  the 
total  amount  of  added  material.  It  is  also  wrong  to  study  the 
swelling  of  jellies  and  to  speak  of  the  results  as  measurements 
of  osmotic  pressure. 

The  low  rate  of  diffusion  of  colloids  was  the  phenomenon  which 
first  started  Graham  on  his  studies  in  regard  to  colloids.  There 
is  no  question  but  that  colloids  do  diffuse.  This  may  be  due  in 
part  to  adsorbed  material,  but  we  also  get  a  diffusion  due  to  the 
Brownian  movements,  since  the  bombardment  by  the  molecules 
of  the  medium  apparently  tends  to  produce  a  uniform  distribu- 
tion of  the  suspended  particles,  except  in  so  far  as  this  is  inter- 
fered with  by  gravity,  surface  tension,  electrical  stresses,  etc. 

1  Van  Bemmelen  Gedenkboek,  152  (.1910). 

2  GLADSTONE:  Phil.  Mag.   (5)  2,  838  (1889);  SABANEEFP:  Jour.  Chem. 
Soc.,  58,   1215  (1890);    REID:  Jour.   Physiology,  31,  438  (1904);    GATIN- 
GRUZEWSKA:  Jour.  Phys.  Chem.  8,  515  (1904);  EULER:  Pflanzenchemie,  2, 
33  (1908):  CHWOLSON:  Jour.  Phys.  Chem.,  16,  88  (1912). 

3  BAYLISS:  Proc.  Roy.  Soc.,  81,  269  (1909);  BILTZ  and  VON  VEGESACK: 
Zeit.  phys.  Chem.,  68,  357  (1909). 


188  APPLIED  COLLOID  CHEMISTRY 

The  dilfusion  of  a  colloid  through  certain  membranes  is  practically 
zero,  while  true  solutions  go  through  readily.  This  property 
has  been  made  use  of  to  purify  colloidal  material  from  dissolved 
substances.  The  process  is  called  dialysis.  In  its  simplest 
form  a  tube,  closed  at  one  end  with  a  parchment  paper  or  animal 
membrane,  is  filled  with  the  solution  to  be  dialyzed  and  is  placed 
in  a  larger  vessel  filled  with  distilled  water.  The  dissolved  sub- 
stances diffuse  out  and  the  colloidal  material  is  left  behind. 

Wintgen1  claims  that  the  reciprocal  of  the  density,  the  specific 
volume,  is  practically  a  linear  function  of  the  percentage  concen- 
tration by  weight.  This  was  tested  with  arsenic  sulphide,  anti- 
mony sulphide,  silicic  acid,  molybdic  acid,  ferric  oxide,  tannin, 
starch,  gelatine,  albumin,  sodium  caseinate  so-called,  and  serum 
globulin.  If  extrapolation  is  made  to  100  percent  colloid  the 
value  for  the  specific  volume  comes  out  too  low  and  conse- 
quently the  value  for  the  density  too  high.  This  is  probably 
due  to  the  existence  of  a  film  of  condensed  water  around  each 
particle.  If  the  suspended  matter  be  in  practically  stable 
equilibrium,  the  density  as  determined  by  the  hydrometer  will 
be  the  same  as  that  determined  by  weighing  a  given  volume.2 
If  the  suspended  material  is  settling  or  rising  with  sufficient 
rapidity,  the  density  shown  by  the  hydrometer  will  be  that  of 
the  liquid  medium.  It  is  all  a  question  of  the  extent  to  which 
the  immersion  of  the  hydrometer  displaces  water  or  displaces 
water  and  suspended  material.  It  is  another  form  of  the  prob- 
lem whether  a  beaker  weighs  more  in  case  a  fly,  which  is  heavier 
than  air,  hovers  in  the  beaker  without  touching  it.  So  long  as 
the  fly  is  there,  the  fly  and  the  air  are  equivalent  to  a  medium 
which  is  denser  than  air  and  consequently  the  downward  thrust 
is  greater  when  the  fly  is  there  than  when  it  is  not.  If  anybody 
doubts  this,  let  him  consider  the  case  where  a  glass  plate  is  laid 
over  the  mouth  of  the  beaker  imprisoning  the  hovering  fly. 

The  product  of  the  specific  volume  into  the  index  of  refraction 
is  said  to  be  a  linear  function  of  the  percentage  composition  by 
weight.3 

1  Kolloidchemische  Beihefte,  7,  251  (1915). 

2  BONDY:  Pogg.  Ann.,  121,  314  (1865);  OWENS:  Geographical  Journal,  37, 
68  (1911);  EHRENBERG:  Die  Bodenkolloide,  84  U915). 

3  WINTGEN:  Kolloidchemische  Beihefte,  7,  268  (1915). 


PROPERTIES  OF  COLLOIDAL  SOLUTIONS  189 


An  anhydrous  substance  in  suspension,  such  as  ferric 
arsenic  sulphide,  gold,  etc.,  appears  to  have  no  effect  on  the  sur- 
face tension  of  the  medium;  but  some  of  the  gelatinous  colloids 
have  a  marked  effect.  Soap  lowers  the  surface  tension  a  great 
deal.  Addition  of  0.5-0.8  g  gelatine  to  100  cc  water  causes  a 
marked  decrease  in  the  surface  tension  of  water,  while  addition- 
of  further  amounts  has  practically  no  effect.2  Addition  of  gum 
arabic  increases  the  surface  tension  within  the  same  limits.  We 
do  not  know  why  this  last  should  be  so. 

It  has  already  been  shown  that  when  oleic  acid  is  dropped  on  a 
water  surface  a  portion  is  adsorbed  and  spreads  over  the  surface, 
while  the  excess,  if  slight  in  amount,  draws  up  into  drops.  The 
thickness  of  the  adsorbed  oil  film  will  vary  from  nothing  up  to 
that  where  the  drop  formation  occurs.  If  water  peptized  oleic 
acid  appreciably,  we  should  find  that,  for  a  given  small  amount 
of  oil,  the  surface  film  would  be  thinner  the  greater  the  peptiz- 
ing  action  of  the  liquid  or  the  greater  the  surface  relatively  to 
the  mass.  Conversely,  if  we  'started  with  a  peptized  mass,  some 
of  the  oil  would  go  into  the  surface  and  form  a  film  there.  This 
has  been  studied  in  some  detail  by  Metcalf,3  who  did  not  realize 
that  he  was  dealing  with  colloidal  solutions.  He  found  that 
peptone  solutions  gradually  formed  an  elastic  surface  film  with 
properties  depending  on  the  temperature.  As  the  rate  of  diffu- 
sion was  very  slow,  this  film  could  be  prepared  more  readily 
by  dropping  a  concentrated  peptone  solution  on  the  surface  of 
pure  water.  The  difference  in  concentration  between  the  sur- 
face and  the  mass  of  the  liquid  was  too  great  to  be  accounted  for 
by  the  Gibbs  formula  for  the  relation  between  change  of  surface 
tension  and  change  of  concentration  in  the  surface  film.  Conse- 
quently Metcalf  assumed  the  existence  of  an  insoluble  form  of 
peptone.  This  cannot  be  the  true  explanation  because  the 
phenomenon  is  general.  The  difficulties  disappear  the  moment 
one  realizes  that  it  is  a  question  of  colloidal  solution  and  not  of  a 
balancing  of  osmotic  pressures.  Saponin4  also  forms  a  markedly 

ILINDER  and  PICTON:  Jour.  Chcm.  Soc.,  60,  72  (1895);  ZLOBICKI:  Ibid., 
87,  1924  (1905). 

2  ZLOBICKI:  Bull.  Acad.  Sci.  Cracovie,  1906,  497. 

3  Zeit.  phys.  Chem.,  62,  1  (1905). 

4  SHORTER:  Phil.  Mag.  (6)  11,  317  (1906);  17,  5GO  (1909). 


190  APPLIED  COLLOID  CHEMISTRY 

viscous  surface  film.  It  is  familiar  to  everybody  that  a  skin 
forms  readily  over  boiled  milk  or  over  cocoa.  The  more  rapid 
formation  after  heating  is  undoubtedly  due  to  the  colloidal  solu- 
tion becoming  less  stable,  though  this  has  not  been  proved  ex- 
perimentally, nor  has  it  yet  been  shown  that  the  film  is  thinner 
the  greater  the  peptizing  action  of  the  liquid. 

Since  the  film  formation  will  occur  more  readily  and  more 
rapidly  if  the  colloidal  substance  adsorbs  air  to  a  marked  extent 
and  since  air  is  always  present  in  actual  experiments,  a  further 
discussion  of  film  formation  from  colloidal  solution  will  be  left 
till  we  take  up  emulsions  and  froths. 

VISCOSITY 

The  viscosity  of  a  liquid  is  the  resistance  offered  to  shearing, 
to  stirring,  or  to  the  flow  through  a  capillary  tube. 1  The  viscos- 
ity coefficient  is  the  force  required  to  move  at  unit  velocity  a 
plate  of  unit  surface  separated  from  another  plate  of  the  same 
size  by  a  layer  of  liquid  of  unit  thickness.  In  the  case  of  water- 
soluble  colloids  the  viscosity  may  increase  enormously  with  the 
concentration,  a  1  percent  solution  of  agar  forming  a  solid 
jelly.  This  is  undoubtedly  in  part  a  question  of  structure  just 
as  we  know  it  is  when  99  percent  of  kerosene  is  emulsified 
in  a  1  percent  of  a  soap  solution.  Here  we  have  minute  glob- 
ules of  oil  coated  by  viscous  soap  films  which  coalesce  to  form 
a  honeycomb  structure.  The  metal  and  the  sulphide  sols  have 
only  a  very  slight  effect  on  the  viscosity  of  water.  This  varies 
with  the  nature  and  the  sub-division  of  the  solid  particles. 
According  to  Hatschek  20-30  percent  of  precipitated  calcium 
carbonate  or  barium  sulphate  does  not  increase  the  viscosity 
very  much.  On  the  other  hand,  Bingham2  found  zero  fluidity 
or  infinite  viscosity  with  4  percent  of  china  clay  or  5.5  percent 
of  graphite. 

Einstein3  has  deduced4  a  formula  for  the  viscosity  (771)  of  a 
suspension  of  rigid  spheres  in  any  liquid,  in  terms  of  the  viscosity 

1  HATSCHEK:  An  Introduction  to  the  Physics  and  Chemistry  of  Colloids, 
21  (1916). 

2  Am.  Chem.  Jour.,  46,  278  (1911);  Jour.  Franklin  Inst.,  181,  845  (1916). 

3  Drude's  Ann.,  19,  289  (,1906). 

4  BURTON:  The  Physical  Properties  of  Colloidal  Solutions,  162  (1916). 


PROPERTIES  OF  COLLOIDAL  SOLUTIONS  191 

(r?)  of  the  pure  liquid  and  the  ratio  (/)  of  the  volume  of  the  sus- 
pended matter  to  the  total  volume, 

171  =  17(1  +  kj) 

where  A;  is  a  constant  which  Einstein  at  first  considered  equal  to 
unity  but  which  he  afterwards  made  equal  to  2.5.  A  similar 
formula  has  been  deduced  by  Hatschek1  for  cases  where  the 
suspended  material  occupies  less  than  40  percent  of  the  total 
volume.  It  differs  from  Einstein's  formula  by  having  k  =  4.5. 
This  formula  was  tested  by  Harrison2  for  starch  and  water  and 
was  found  to  hold  fairly  well  up  to  30  percent  concentration. 
According  to  these  formulas,  the  viscosity  depends  only  on  the 
volume  of  the  dispersed  phase  and  not  at  all  upon  the  degree  of 
dispersity.  Oden3  found,  however,  an  approximately  50  per- 
cent greater  viscosity  with  sulphur  sols  in  which  the  particles 
had  a  diameter  of  about  10,uju  than  with  sols  in  which  the  sulphur 
particles  had  a  diameter  of  about  lOOju/*.  Hatschek4  attributes 
this  very  properly  to  the  existence  of  an  adsorption  film  of  liquid 
around  the  sulphur  particles,  a  phenomenon  which  was  not 
considered  when  the  formula  was  deduced.  On  the  assumption 
that  the  thickness  of  the  adsorbed  film  is  independent  of  the  size 
of  the  particles,  Hatschek  deduces  that  the  thickness  is  about 
0.87ju/z  which  corresponds  to  an  increase  in  volume  of  about 
62  percent  for  particles  having  a  diameter  of  10^. 

Hatschek5  has  also  deduced  a  formula  for  the  viscosity  of  two 
liquid  phases  in  which  the  volume  of  the  dispersed  phase  is  the 
larger, 


where  A  =  1/f  and  is  the  ratio  of  the  total  volume  to  the  volume 
of  the  dispersed  phase.  From  this  formula  he  deduces  that  one 
volume  of  caseine  binds  more  than  eleven  volumes  of  water  and 
that  in  a  0.4  —  0.5  percent  rubber  sol,  the  rubber  binds  75-100 
volumes  of  liquid. 

1  Zeit.  Kolloidchemie,  7,  301  (1910). 

2  Jour.  Soc.  Dyers  and  Colorists,  27,  April  (1911). 

3  Zeit.  phys.  Chem.,  80,  709  (1912). 

4  Zeit.  Kolloidchemie,  11,  280  (1912). 

o  Ibid.,  8,  34  (1911);  11,  284  (1912);  12,  238  (1913). 


192  APPLIED  COLLOID  CHEMISTRY 

We  get  rather  curious  results  if  we  apply  Hatschek's  views  on 
viscosity  to  Bingham's  experiments  on  zero  fluidity.  If  we  make 
the  assumption  that  plastic  flow  is  reached  when  the  surfaces 
of  adsorbed  water  are  in  contact  and  if  we  make  the  further  as- 
sumption that  we  are  dealing  with  spheres  in  open  piling,  the 
voids  will  then  be  48  percent  of  the  whole  and,  in  the  case  of 
graphite  for  instance,  the  amount  of  water  adsorbed  by  the  graph- 
ite must  be  94.5  —  48  =  47.5  volume  percent,  or  each  volume  of 
graphite  must  adsorb  about  nine  volumes  of  water.  If  we  assume 
close  piling  or  different  sizes  of  graphite  powder,  the  voids  will 
be  less  and  the  amount  of  water  to  be  adsorbed  will  be  greater. 
Since  the  volumes  of  two  spheres  are  proportional  to  the  cubes  of 
the  radii,  one  volume  of  graphite  will  hold  seven  volumes  of  water 
if  the  thickness  of  the  water  film  is  equal  to  the  radius  of  the 
graphite  particles.  If  the  thickness  of  the  water  film  is  1.2  times 
the  radius,  the  graphite  will  hold  eleven  volumes  of  water.  This 
is  the  same  type  of  calculation  that  Hatschek  made  and  it 
shows  that  it  is  theoretically  possible  on  this  assumption  to 
account  for  zero  fluidity  in  a  graphite-water  mixture  containing 
5.5  percent  graphite.  It  has  not  been  shown,  however,  that 
the  adsorbed  films  of  water  on  the  graphite  particles  are  of  the 
desired  thickness,  nor  has  it  been  shown  that  47  percent  of  the 
water  in  the  mixture  is  in  a  different  state  from  the  rest  of  the 
water.  It  might  be  possible  to  do  this  last  by  measuring  the 
vapor  pressure  curve  for  graphite- water  mixtures. 

If  the  suspended  particles  aggregate  into  chains,  the  viscosity 
will  be  increased  very  much.  If  the  particles  form  larger  spheri- 
cal particles  which  are  homogeneous,  there  will  be  a  decrease  in 
the  viscosity,  because  of  the  decrease  in  the  surface  and  conse- 
quently in  the  amount  of  bound  water.  If,  however,  the  particles 
simply  agglomerate  loosely  into  spherical  masses,  the  viscosity 
will  increase  because  the  water  in  the  voids  inside  the  spherical 
agglomerates  no  longer  counts  as  free  water.  We  shall  therefore 
expect  to  get  an  increase  of  viscosity  as  a  result  of  agglomeration 
when  the  effect  of  agglomeration  is  not  to  increase  the  size  of 
homogeneous  drops. 

One  very  interesting  case  of  the  effect  of  viscosity  and  pre- 
sumably of  the  adsorbed  water  film  occurs  in  the  settling  of 
slimes  or  pulps.  Here  the  particles  vary  in  size  from  those 


PROPERTIES  OF  COLLOIDAL  SOLUTIONS  193 

coarse  enough  to  settle  moderately  rapidly  down  to  those  so 
fine  that  the  Brownian  movements  tend  to  keep  them  up  com- 
pletely. If  there  is  a  large  amount  of  water  present  and  the 
pulps  are  consequently  very  dilute,  each  particle  separates 
practically  unaffected  by  the  others.  The  upper  part  of  the 
liquid  remains  cloudy  owing  to  the  presence  of  the  very  fine  parti- 
cles, there  is  no  sharp  line  of  demarcation  in  the  liquid,  and  there 
is,  or  may  be,  a  sharp  line  at  the  bottom  separating  the  coarser 
particles  which  have  settled  and  the  supernatant  liquid.1  With 
increasing  concentration  the  coarser  particles  tend  to  interfere 
with  the  finer  ones  and  to  carry  them  down.  The  suspended 
pulp  seems  to  settle  as  a  whole,  the  upper  portion  of  the  liquid  is 
practically  clear  with  a  sharp  line  separating  it  from  the  settling 
mud,  and  there  is  no  visible  dividing  line  at  the  bottom  of  the 
vessel  between  the  slimes  which  have  settled  and  those  which  are 
settling.  According  to  Free  the  change  from  one  type  to  the 
other  came  at  about  8-9  percent  with  the  kaolin  suspension  that 
he  took,  the  higher  concentrations  giving  what  he  calls  consolida- 
tion settling  and  the  lower  concentrations  subsidence  settling. 

When  a  given  volume  of  pulp  settles,  an  equal  volume  of  water 
must  rise  and  when  the  pulp  is  fairly  viscous,  it  acts  practically  as  a 
filter,  removing  all  the  finest  particles.2  Deane  says  that  the  effect 
of  reducing  the  resistance  to  the  upward  flow  of  the  water  by 
creating  a  channel  for  the  passage  of  the  liquid  can  be  shown 
very  well  by  placing  a  sample  of  pulp  in  two  test-tubes,  one  of 
which  is  held  vertically,  while  the  other  is  inclined  slightly. 
In-addition  to  the  increased  clarification  in  the  inclined  tube  due 
to  the  increased  settling  area,  it  will  be  noticed  that  the  pulp  in 
settling  away  from  the  upper  surface  of  the  tube  leaves  an  open 
channel  which  facilitates  the  free  passage  of  the  liquid.  The 
rising  current  of  the  liquid  on  the  upward  surface  and  the  settling 
of  the  solid  on  the  lower  surface  is  quite  marked.  If  wires  are 
placed  vertically  in  a  tank  and  are  given  rapid  vibrations  of  low 
amplitude,  this  will  cause  channelling  and  will  hasten  thickening 
by  making  the  upward  flow  of  the  liquid  easier. 

The  dairymen  have  noticed  that,  contrary  to  what  one  would 

1  OWENS:  Geographical  Journal,  37,  68  U911);  FREE:  Eng.  Min.  Jour.,  109, 
601  (1916). 

2  DEANE:  Trans.  Am.  Electrochem.  Soc.,  37  (1920). 

13 


194  APPLIED  COLLOID  CHEMISTRY 

expect,  cream  rises  more  rapidly  and  more  completely  in  a  deep 
vessel  than  in  a  shallow  pan.1  The  finer  particles  of  butter  fat 
are  caught  by  the  coarser  ones  and  swept  upwards  because  the 
concentration  of  the  coarser  particles  per  unit  of  cross-section 
soon  becomes  high  enough  to  have  a  filtering  effect.  The  differ- 
ence between  this  and  the  ore  pulp  is  that  the  cream  rises  and 
filters  upwards,  while  the  pulp  settles  and  filters  downwards. 
In  neither  case  has  anybody  made  any  experiments  as  to  any 
possible  relation  between  the  concentration  at  which  the  filtra- 
tion becomes  effective  and  the  concentration  for  zero  fluidity. 

Arisz2  has  made  a  number  of  valuable  experiments  on  the  vis- 
cosity of  gelatine  peptized  at  50°-60°  by  glycerine  of  1.176 
specific  gravity,  corresponding  to  about  32  percent  of  water  in 
the  glycerine.  He  finds  that  above  65°  there  is  a  continuous 
decrease  in  the  viscosity  of  the  gelatine  sols.  At  95°  the  viscosity 
of  a  10  percent  sol  dropped  in  thirty  minutes  to  92  percent  of 
the  viscosity  which  it  had  three  minutes  after  reaching  95°.  At 
85°  the  drop  was  only  to  97  percent  in  thirty  minutes  and  at  75° 
to  99  percent.  At  65°  there  was  no  change  in  viscosity  in 
twenty-four  hours.  Arisz  believes,  apparently  properly,  that 
the  change  above  65°  is  due  to  a  progressive  decomposition  of  the 
gelatine  which  becomes  negligible  at  and  below  this  temperature. 
At  65°  equilibrium  is  reached  in  less  than  three  minutes  and 
at  lower  temperatures  with  ever-increasing  slowness.  For  10 
percent  gelatine  in  the  standard  glycerine,  Arisz  considers  that 
the  equilibrium  values  for  the  viscosity  as  determined  in  the 
Ostwald  viscosimeter  are:  222  at  65°;  415  at  55°;  about  950  at 
50°;  less  than  4200  at  47°;  about  5000  at  46°;  and  more  than 
30,000  at  44°,  which  is  about  the  temperature  at  which  the  sol 
sets  to  a  jelly.  Since  equilibrium  is  reached  slowly  at  the  lower 
temperatures,  the  viscosity  of  the  sol  will  be  less  than  the  equilib- 
rium value  if  the  sol  is  cooled  say  from  65°  to  55°  and  the  viscos- 
ity will  increase  with  time.  If  a  sol  in  equilibrium  at  say  46°  is 
heated  to  55°,  the  viscosity  will  be  greater  than  the  equilibrium 
value  and  will  decrease  with  increasing  time.  Until  equilibrium 
is  reached,  the  viscosity  of  a  given  gelatine-glycerine  sol  will  thus 

1  McKAY    and   LARSEN:  Principles  and  Practice  of  Butter-making,   125 
(1906). 

2  Kolloidchemische  Beihefte,  7,  1  (1915). 


PROPERTIES  OF  COLLOIDAL  SOLUTIONS  195 

depend  at  temperatures  below  65°  on  the  previous  history  of  the 
sample  and  on  the  time  that  it  has  been  held  at  the  temperature 
in  question. 

These  gelatine  sols  show  the  Tyndall  beam  and  Arisz  finds 
that  the  intensity  of  the  scattered  light  increases  with  falling 
temperature  and  with  increasing  time,  provided  the  viscosity  is 
below  the  equilibrium  value.  Cooling  a  1  percent  gelatine- 
glycerine  sol  from  70°  to  20°  caused  a  four-fold  increase  in  the 
intensity  of  the  scattered  light.  When  a  similar  solution  was 
cooled  to  18°  and  held  there,  the  intensity  of  the  scattered  light 
at  the  end  of  nine  days  was  nearly  six  times  the  initial  value  at 
18°.  There  is  therefore  a  parallelism  between  the  changes  in  the 
Tyndall  effect  and  the  changes  in  the  viscosity  of  these  sols,  and 
consequently  part  at  least  of  the  viscosity  changes  must  be  due 
to  an  agglomeration  of  the  particles.  Arisz  considers  that  the 
small  drops  or  particles  have  run  together  to  form  larger  drops; 
but  this  cannot  be  true  because  that  would  mean  a  decrease  in 
the  viscosity  and  not  an  increase.  The  increase  in  the  apparent 
size  of  the  particles  which  one  must  assume  in  order  to  account 
for  the  Tyndall  effect  can  be  due  equally  well  to  an  agglomeration 
of  particles  or  drops  without  having  them  coalesce  to  a  homo- 
geneous particle  or  drop.  That  this  is  what  actually  happens  is 
made  probable  by  the  experiments  of  Meyer1  on  starch.  This  of 
course  does  not  exclude  the  possibility  of  viscosity  changes  due 
to  displacement  of  equilibrium  between  two  forms  of  gelatine2 
in  case  these  exist.  The  important  point  is  that  the  increase  in 
viscosity  does  go  hand  in  hand  with  an  increase  in  agglomeration 
which'must  mean  that  increasing  agglomeration  involves  decrease 
in  the  amount  of  available  free  water.  The  phenomenon  is 
apparently  general  because  .Freundlich3  has  made  use  of  the 
increase  in  viscosity  as  a  means  of  studying  the  rate  of  agglomera- 
tion of  alumina  sols. 

With  colloidal  solutions  of  gelatine  and  other  substances  of  the 
same  type,  the  viscosity  changes  with  the  time,  showing  that 
there  is  a  gradual  change  in  structure.  This  is  further  confirmed 
by  the  fact  that  the  viscosity  of  such  solutions  changes  when  they 

1  Kolloidchemische  Beihefte,  5,  1  (1913). 

2 SMITH:  Jour.  Am.  Chem.  Soc.,  41,  135  (1919). 

3  Trans.  Faraday  Soc.,  9,  66  (1913). 


196  APPLIED  COLLOID  CHEMISTRY 

are  shaken  violently.  Until  we  know  more  than  we  do  now  about 
the  physical  state  of  these  colloidal  solutions,  it  is  distinctly 
unwise  to  predict  anything  very  definite  about  their  viscosities. 
Where  a  film  formation  is  possible,  the  surface  viscosity  will 
be  quite  different  from  that  in  the  mass  of  the  liquid.  This  is 
very  striking  in  the  case  of  colloidal  solutions  of  saponin,  peptone, 
etc.  With  saponin  the  surface  viscosity  is  60  times  that  of  the 
mass  of  the  liquid.1  With  peptone  the  film  is  so  solid  that  the 
surface  viscosity  cannot  be  measured. 

COLOR 

It  will  simplify  matters  not  to  limit  the  discussion  to  the  colors 
of  colloidal  solutions,  but  to  consider  something  about  the 
colors  of  colloids  in  general.  The  colors  which  depend  on  phys- 
ical structure  and  not  on  pigments  are  known  as  structural  colors 
and  are  often  connected  with  the  presence  of  colloidal  material. 
An  absolutely  smooth  reflecting  surface  is  invisible.  It  is  only 
when  the  specular  reflection  is  changed  to  some  extent  to  diffuse 
reflection  that  we  see  the  surface/It  was  suggested  that  the 
Zeppelins  should  be  coated  with  a  polished  surface  of  metallic 
chromium  so  as  to  make  them  invisible.  While  it  would  be 
impossible  to  see  such  a  surface,  almost  anybody  would  realize 
that  something  queer  was  going  on  if  he  should  happen  to  see  a 
reflected  portion  of  the  earth's  surface  apparently  moving  along 
in  the  sky./ While  this  last  might  never  happen,  the  reflection 
of  light  from  the  polished  surface  would  more  than  counter- 
balance the  advantages  of  the  theoretical  invisibility. 

A  substance  is  also  invisible  if  there  is  no  reflection  or  refraction 
at  its  surfaces.  If  we  suspend  a  transparent  powder  in  a  trans- 
parent liquid  having  the  same  index  of  refraction,  the  powder  will 
be  invisible.  Some  glass  powders  in  carbon  bisulphide  are  practi- 
cally invisible.  If  a  glass  rod  is  dipped  into  a  mixture  of  olive 
oil  and  cassia  oil,  the  rod  appears  only  to  reach  to  the  surface  of 
the  liquid  for  it  cannot  be  seen  in  the  liquid.2  It  may  happen, 
however,  that  the  dispersion  of  the  liquid  mixture  differs  from 
that  of  the  solid,  in  which  case  there  will  be  an  agreement  of 

1  MALCOLM:  Phil.  Mag.  (6)  12,  508  (1906). 

2  VON  BEZOLD:  Die  Farbenlehre,  62  (1874). 


PROPERTIES  OF  COLLOIDAL  SOLUTIONS  197 

refractive  index  only  for  certain  wave-lengths,  which  will  be 
transmitted.  We  may  therefore  get  the  transmission  of  colored 
light,  a  phenomenon  first  studied  by  Christiansen.1  When 
clean  powdered  glass  is  immersed  in  carbon  bisulphide  and  ben- 
zene is  added,  the  transmitted  light  is  first  red,  then  yellow, 
green,  and  blue  in  succession.  For  the  same  mixture,  the  trans- 
mitted light  changes  towards  the  blue  with  rising  temperature.2 
In  some  cases  the  light  is  extremely  pure,  the  range  of  ref  rangibility 
being  only  two  and  a  half  times  that  included  by  the  two  D  lines.3 

If  we  have  a  thin  film  with  light  reflected  from  the  front  and  the 
back  surfaces,  it  may  happen  that  the  crest  of  certain  waves 
reflected  from  one  surface  will  coincide  with  the  hollow  of  waves 
reflected  from  the  other  surface,  in  which  case  this  particular 
color  will  disappear  and  the  place  where  this  disappearance  oc- 
curs will  appear  to  have  the  complementary  color.  The  colors  of 
thin  films4  are  often  known  as  Newton's  Rings.  We  are  quite 
familiar  with  them  in  the  case  of  soap  bubbles  and  with  oil  films 
on  the  ground.  In  the  streets  of  Washington  the  colors  of  the 
oil  films  are  exceptionally  brilliant;  but  this  is  not  due  to  any 
particular  brand  of  oil  which  is  spilled  there.  It  is  because  the 
streets  are  asphalted  and  consequently  there  is  practically  no 
reflection  of  light  from  the  asphalt  surface  to  interfere  with  the 
colors  of  thin  films.  This  is  a  very  good  illustration  of  the  im- 
portance of  a  dark  background  in  the  case  of  structural  colors. 
The  colors  of  tempered  steel,  of  lead  skimmings,  of  Nobili's 
metallochromes,  and  the  flashing  colors  of  certain  insects'  wings 
are  further  illustrations  of  the  colors  of  thin  plates.  If  black  ice 
is  struck  with  a  heavy  stick,  the  colors  of  thin  plates  appear  in 
great  beauty  at  the  fractured  surfaces. 

We  may  get  colors  by  reflection  and  refraction.  The  rainbow 
is  an  illustration  of  this.  The  old-fashioned  cut-glass  chandelier 
gives  a  good  example  of  the  colors  that  may  be  produced  by 
light  passing  through  a  prism.  If  we  have  a  number  of  lines  ruled 
parallel  and  very  close  together  on  a  smooth  surface,  this  gives  us 
what  is  known  as  a  diffraction  grating,  which  may  spread  a  ray 

1  Wied.  Ann.,  23,  289  (1884). 

2  WOOD:  Physical  Optics,  110  (1911). 
SRAYLEIGH:  Proc.  Roy.  Inst.,  16,  116  (1889). 
*  TTNDALL:  Electricity  and  Light,  97  (1895). 


198  APPLIED  COLLOID  CHEMISTRY 

of  light  out  in  a  way  very  similar  to  a  prism.  Agates  are  com- 
posed of  thin  films  about  0.001  of  a  millimeter  in  thickness,  and 
when  the  agate  is  properly  cut  it  may  behave  like  a  diffraction 
grating.1  In  the  case  of  mother-of-pearl,  we  have  alternate 
layers  of  calcium  carbonate  and  an  organic  material.  These  layers 
overlap  very  slightly  in  a  regular  fashion  and  consequently  act 
to  some  extent  like  a  diffraction  grating,  giving  rise  to  the  peculiar 
effect  known  as  pearly  lustre.2 

The  reflection  of  light  by  powdered  material  may  give  rise  to  a 
number  of  interesting  color  phenomena.  When  white  light  is 
scattered  from  a  surface  instead  of  being  reflected  as  in  a  mirror, 
it  gives  rise  to  the  sensation  of  white.  A  block  of  ice  is  not  white, 
because  it  does  not  scatter  the  light.  If  the  ice  is  powdered,  or 
still  better  if  we  have  snow,  the  light  is  scattered  and  we  call  the 
snow  white.  Instead  of  having  solid  particles  of  ice  in  air  we 
may  have  air  bubbles  embedded  in  liquid,  and  then  we  get  a  white 
froth  or  foam.  If  the  blue  crystals  of  copper  sulphate  are  ground 
to  a  fine  powder,  the  light  passes  through  such  fine  layers  of  the 
material  that  it  does  not  become  colored  blue  to  any  appreciable 
extent.  On  the  other  hand,  the  light  is  scattered  from  the  pow- 
dered surfaces,  and  the  powdered  copper  sulphate  looks  white 
instead  of  blue.  When  silver  is  precipitated  by  an  electric 
current,  it  does  not  come  down  with  a  mirror-like  surface,  but 
rather  in  a  mass  of  tiny  crystals  which  scatter  the  light  in  every 
direction.  Consequently  electrolytic  silver  is  white  and  only 
has  the  characteristic  appearance  of  silver  after  it  has  been  bur- 
nished. There  is  no  white  pigment  in  any  white  flowers,  or  in 
white  hair,  and  probably  not  in  white  bark.  The  white  color  of 
the  lily  is  due  to  the  presence  of  innumerable  air  bubbles  and  the 
same  is  true  of  white  hair.  In  order  that  hair  may  turn  white  in 
a  single  night,  it  is  not  necessary  for  the  original  pigment  to 
disappear  and  for  white  pigment  to  develop.  It  is  only  necessary 
for  a  mass  of  minute  air  bubbles  to  be  formed  in  the  hair  as  a 
result  of  worry.  While  this  accounts  for  the  physical  change,  we 
are  as  much  at  a  loss  as  ever  to  know  why  intense  grief  should 
develop  air  bubbles  in  the  hair. 

The  scattering  of  light  by  a  powder  or  by  the  fibers  of  a  sheet 

1  GOODCHILD:  Precious  Stones,  165  (1908). 
2Pfund:  Jour.  Franklin  Inst.  183,  453  (1917). 


PROPERTIES  OF  COLLOIDAL  SOLUTIONS  199 

of  paper  is  due  to  the  difference  in  the  index  of  refraction  between 
the  solid  particles  and  the  air.  If  we  replace  the  air  by  some 
liquid  having  very  nearly  the  same  index  of  refraction  as  the  solid 
particles,  these  latter  will  cease  to  be  white  and  will  become  more 
transparent.  It  is  well  known  that  oiled  paper  is  distinctly 
translucent.  If  powdered  copper  sulphate  were  immersed  in  a 
liquid  having  about  the  same  index  of  refraction  as  the  crystals, 
these  would  probably  become  blue.  This  experiment  has  not 
been  tried;  but  there  is  a  more  interesting  one  which  has  puzzled 
people  for  a  good  while.  In  the  zoological  gardens  some  flamingoes 
are  much  redder  than  others.  At  one  time  it  was  thought  that 
the  redness  was  connected  with  a  fish  diet;  but  flamingoes  have 
been  raised  which  had  bright  red  feathers  even  though  the  birds 
had  never  eaten  fish,  so  this  explanation  had  to  be  given  up. 
It  is  now  believed  that  the  brilliancy  of  the  color  is  due  to  the 
presence  of  oil  in  the  feathers,  this  making  them  more  translucent 
and  bringing  out  the  red  color.1  Water  colors  always  become 
paler  as  the  water  dries  out,  because  more  light  is  then  scattered. 
Some  people  have  gone  so  far  as  to  say  that  white  pigments  do  not 
occur  in  nature  in  animals  or  plants;2  but  this  is  an  over-state- 
ment. Some  butterflies  have  a  whitish  pigment  in  their  wings3 
and  the  silvery  whiteness4  of  many  fishes  is  due  to  the  presence  of 
a  substance  called  guanin,  and  riot  to  air  bubbles. 

Complete  absorption  of  light  gives  the  sensation  known  as 
black.  Pulverulent  silver  is  black  because  the  crystals  are  so 
arranged  that  all  the  light  is  absorbed  by  reflection  forward  and 
back.  This  can  be  shown  in  another  way.  If  we  take  a  bunch  of 
steel  needles  and  place  them  with  their  points  side  by  side  facing 
the  observer,  the  mass  will  appear  black,5  although  we  know  that 
no  single  needle  is  black  in  itself.  The  rich  color  of  velvet  is  due 
to  its  trapping  the  light,  and  the  peculiar  effect  known  as  damask 
is  due  to  the  fibers  being  arranged  in  two  different  ways,  so  that 
one  set  absorbs  more  light  than  the  other.  Trees  and  grass  may 
act  as  light  traps  when  seen  from  above  and  they  are  said  to  look 

1  KRUKENBERG:  Vergleichend-physiologische  Studien,  1  V,  90  (1887). 

2  GADOW:  Proc.  Zool.  Soc.,  London,  1882,  410. 

3  HOPKINS:  Phil.  Trans.,  186B,  661  (1895). 

4  NEWBIGIN:  Color  in  Nature,  37,  219  (1898). 

5  WOOD:  Physical  Optics,  449  (1911).  • 


200  APPLIED  COLLOID  CHEMISTRY 

black  to  aviators.1  If  we  have  a  material  which  absorbs  light 
so  completely  that  the  powder  is  black,  we  shall  get  all  gradations 
between  the  original  color  and  black  if  we  grind  up  such  a  sub- 
stance, just  as  we  got  all  variations  between  blue  and  white  by 
grinding  crystals  of  copper  sulphate.  If  we  start  with  yellow  gold 
and  make  it  more  porous,  it  will  become  brown  before  becoming 
black.  This  happens  unintentionally  in  the  assaying  of  gold 
when  a  brown  powder  is  obtained  which  people  have  thought  was 
an  allotropic  form  of  the  metal.  It  is  merely  a  porous  gold  and 
when  it  is  heated  until  the  crystals  sinter  together  and  become 
less  porous,  the  yellow  color  of  gold  reappears. 

If  we  have  very  fine  particles  suspended  in  a  transparent  solid, 
liquid,  or  gas,  these  particles  scatter  blue  light  much  more  than 
they  do  red  light,  and  consequently  such  a  mass  appears  red 
by  transmitted  light  and  blue  by  reflected  light.  Skimmed  milk 
is  an  instance  of  this  sort,  being  distinctly  bluish  by  reflected 
light  and  reddish  by  transmitted  light.  Tobacco  smoke  is  blue 
by  reflected  light  and  red  by  transmitted  light.  The  blue  color 
of  the  sky  is  due  to  light  which  is  scattered  by  drops  of  liquid  or 
by  particles  of  dust  in  the  air.  The  intensity  of  the  color  is  un- 
doubtedly increased  by  the  fact  that  we  see  this  against  the  black 
background  of  infinite  space.  The  sunset  colors  are  due  to  the 
light  which  is  transmitted  and  not  scattered. 

There  is  no  blue  pigment  in  blue  eyes  and  it  was  pointed  out  by 
Tyndall2  years  ago  that  the  blue  of  the  eye  is  really  the  blue  of 
turbid  media,  and  is  thus  analogous  to  the  blue  of  the  sky  or  the 
blue  of  skimmed  milk.  At  the  back  of  the  iris  there  is  a  dark  pig- 
ment known  as  the  uvea,  which  prevents  the  reflection  of  light 
and  prevents  the  color  of  the  blood  from  being  seen.  When 
this  dark  pigment  is  absent,  we  have  an  albino  with  a  pink  eye. 
The  various  stages  between  the  blue  and  the  gray  eye  are  due  to 
differences  in  the  coarseness  of  the  particles  giving  rise  to  the 
blue  color,  the  blue  color  being  the  most  intense  the  finer  the 
particles.  This  is  probably  the  reason  why  babies'  eyes  are  so 
very  blue,  because  the  suspended  particles  tend  to  grow  coarser 
with  increasing  age.  All  other  people  have  a  yellowish-brown 
pigment  in  the  front  of  the  iris  and  the  combination  of  the  struc- 


:  Jour.  Franklin  Inst.,  187,  303  (1919). 
2  Fragments  of  Science  :  The  Sky. 


PROPERTIES  OF  COLLOIDAL  SOLUTIONS  201 

tural  blue  with  the  yellowish-brown  pigment  gives  rise  to  the  green, 
hazel,  brown  and  black  eyes.  Except  with  people  who  have  very 
black  eyes,  the  pigment  in  the  front  of  the  iris  does  not  develop 
at  birth,  just  as  the  teeth  do  not  come  till  later.  Consequently 
many  infants  have  blue  eyes,  which  change  to  hazel,  brown  or 
black  as  they  grow  older,  while  the  reverse  change  never  occurs. 
f  Once  in  a  while  we  see  a  man  whose  eyes  are  colored  differently^ 
This  means  that  pigment  has  developed  in  one  eye  and  not  in  the 
other,  or  that  the  pigment  has  developed  unequally  in  the  two 
eyes. 

The  green  eye  is  due  to  the  combination  of  structural  blue  with 
the  pigment  yellow.  While  this  is  not  a  common  type  in  human 
beings,  we  get  it  very  markedly  in  the  case  of  the  green  tree-frog, 
who  has  no  green  pigment  and  whose  green  color  is  due  to  struc- 
tural blue  with  a  yellow  pigment  overlaying  it.  If  we  scrape 
the  pigment  layer  off  the  back  of  an  unfortunate  frog,  he  turns 
blue.  In  fact,  the  grass-green  Australian  tree-frog  is  called 
Hyla  coerulea,  because  he  is  blue  when  he  comes  to  us  preserved 
in  alcohol,  the  alcohol  having  dissolved  the  yellow  pigment. 

There  is  a  wide-spread  belief  that  people  with  blue  or  gray  eyes 
are  better  rifle  shots  than  people  with  dark  eyes.  Since  we  do 
not  see  through  the  iris,  this  could  not  be  cause  and  effect,  though 
it  is  theoretically  possible  that  the  two  characteristics  might  be 
correlated.  Colonel  Chamberlain  has  made  a  study  of  tjiis 
matter  and  finds  that  of  three  thousand  soldiers  who  had  qualified 
for  the  grade  of  marksman  or  better,  approximately  two  thousand 
had  blue  or  gray  eyes,  a  result  which  seems  at  first  to  bear  out 
the  popular  belief.  He  finds  however  that  of  one  hundred 
thousand  white  soldiers  approximately  two  thirds  have  blue  or 
gray  eyes.  People  have  made  the  mistake  of  considering  abso- 
lute figures  and  not  percentage. 

If  we  leave  out  of  account  feathers  with  a  blue  metallic  lustre, 
which  may  constitute  a  special  case,  we  may  say  that  there  is  no 
blue  pigment  in  the  feathers  of  any  bird.  This  means  that  the 
blue  of  the  kingfisher,  the  indigo  bunting,  the  blue-jay,  and  the 
blue-bird,  is  not  due  to  blue  pigment;  but  is  a  structural  color. 
The  only  pigment  in  the  blue  feathers  of  these  birds  is  a  dark 
brown  one  which  apparently  serves  merely  as  a  background,  just 
as  in  the  case  of  the  blue  eyes.  The  best  explanation  of  the  blue 


202  APPLIED  COLLOID  CHEMISTRY 

feather1  is  that  the  horny  matter  is  filled  with  an  enormous  num- 
ber of  minute  air  bubbles,  which  scatter  blue  light  and  transmit 
red.  The  blue  feather  is  therefore  the  same  in  principle  as,  or 
the  opposite  in  detail  from,  the  blue  sky.  The  color  of  the  sky 
is  due  to  the  scattering  of  light  by  particles  of  liquid  or  solid 
suspended  in  a  gas,  the  air.  The  blue  of  the  feathers  is  due  to  the 
scattering  of  light  by  bubbles  of  gas,  air,  suspended  in  a  solid 
medium.  While  this  explanation  is  undoubtedly  the  right  one, 
nothing  analogous  to  a  blue  feather  has  yet  been  made  in  the 
laboratory.  This  is  one  of  the  things  that  people  must  do  in  the 
future.  Apparently  nobody  has  made  any  experiments  to  de- 
termine whether  the  blue  light  from  the  blue  eyes  and  the  blue 
feathers  is  polarized.  This  should  be  the  case. 

In  this  country  the  tufted  titmouse  is  a  gray  bird  with  no 
brilliant  colors;  but  the  German  titmice  show  great  differences  in 
color.  One  variety  is  green  on  the  back,  due  to  structural  blue 
overlaid  by  a  pigment  yellow,  and  yellow  on  the  belly.  In 
another  variety  the  brown  pigment  which  is  essential  to  the 
structural  blue  has  not  developed  and  the  bird  is  consequently 
yellow  all  over.  There  is  a  third  form  in  which  the  yellow  pig- 
ment has  not  developed  and  the  bird  is  therefore  blue  on  the 
back  and  white  underneath.  One  would  like  to  think  that  the 
differences  in  color  between  the  male  and  the  female  redstart 
were  due  to  the  presence  in  the  male  of  a  single  color  which  was 
lacking  in  the  female;  but  this  cannot  be  the  case,  unless  it  can 
be  shown  that  the  black  on  the  male  redstart  is  due  to  a  red 
pigment  just  as  hematite  is  black  in  mass. 

Tyndall,2  Aitken3  and  Spring4  consider  that  the  color  of  water 
is  blue,  while  Rayleigh5  believes  that  the  blue  of  the  sea  is 
merely  the  light  reflected  from  the  blue  sky.  Assuming  that 
water  is  in  itself  blue,  it  would  look  black  if  no  light  were  re- 
flected back  from  the  interior.  A  difference  in  the  index  of 
refraction  due  to  difference  of  temperature  might  be  sufficient  ; 


Zool.  Jahrbiicher,  Abt.  syst.   Geog.  Biol.   Thiere,   16,  267 
(1902). 

2  Fragments  of  Science  :  Voyage  to  Algeria. 

3  Proc.  Roy.  Soc.  Edin.,  11,  472  (1882). 

4  Bull.  Acad.  roy.  belg.  (3)  5,  55  (1883);  12,  814  (1886). 
6  Nature,  83,  48  (1910). 


PROPERTIES  OF  COLLOIDAL  SOLUTIONS  203 

but  a  more  usual  cause  is  the  presence  in  suspension  of  reflecting 
particles.  If  these  are  few  in  number  the  water  will  appear 
indigo  to  blue.  If  they  are  many  in  number  we  shall  begin  to 
get  the  yellowish  to  red  color  of  light  transmitted  through  a 
turbid  medium  and  the  water  will  appear  green.  Muddy  waters 
may  appear  yellow  or  brown.  As  a  matter  of  fact,  the  deep  sea 
which  contains  little  suspended  matter,  is  blue,  while  near  shore 
where  there  is  more  suspended  matter,  the  color  is  often  a 
brilliant  green,  especially  in  the  tropics.  The  presence  of  air 
bubbles  will  act  in  the  same  way  and  in  an  indigo  sea  the  crest 
of  a  wave  may  be  green.  The  Lake  of  Constance  is  quite  green 
because  of  the  suspended  calcium  carbonate,  while  the  Lake  of 
Geneva  is  more  blue  because  there  is  less  suspended  matter. 
The  Rhine  at  Strasbourg  is  green  and  contains  70  percent  more 
calcium  carbonate  than  the  blue  Rhone  at  Geneva.  The  Gulf 
Stream  is  blue  because  there  is  less  carbon  dioxide  in  the  warm 
water  and  consequently  less  calcium  carbonate.  The  clear 
brown  color  of  some  New  England  brooks  is  probably  due  to  the 
presence  of  tannin-like  materials. 

The  colloidal  metals  are  a  special  and  very  interesting  case  of 
the  colors  of  colloidal  solutions.  Colloidal  gold  solutions1  can  be 
prepared  which  are  red,  violet  or  blue  by  transmitted  light;  and 
colloidal  silver  solutions2  which  are  yellow,  red  or  blue.  In 
general  the  blue  gold  solutions  contain  coarser  particles  than  the 
red  ones,  but  this  is  not  always  true.  The  peculiarities  in  thp 
colors  of  colloidal  metals  seem  to  be  connected  intimately  with 
the  fact  that  the  metals  show  selective  reflection.  In  the  case  of 
selective  reflection,  some  or  all  of  the  light  which  is  adsorbed 
very  strongly  by  a  given  substance  is  reflected  strongly  from  a 
polished  surface  of  that  substance.  The  surface  color  of  magenta 
is  green,  while  the  body  color  or  the  color  due  to  the  transmitted 
light  is  red.  With  indigo  the  surface  color  is  red  and  the  body 
color  blue.  The  surface  color  is  believed  to  be  due  to  resonance, 
the  substance  emitting  the  rays  which  it  adsorbs  very  strongly. 
It  seems  probable  that  a  very  small  granular  particle  might  emit 
surface  color  by  resonance  on  the  opposite  side  from  the  source 
of  light,  in  which  case  we  should  have  the  surprising  phenomenon 

1  FARADAY:  Phil.  Trans.,  147,  145  (1857). 

2  CAREY  LEA:  Am.  Jour.  Sci.  (3)  37,  475  (1889), 


204  APPLIED  COLLOID  CHEMISTRY 

of  a  substance  apparently  transmitting  the  light  which  it  adsorbs 
most  strongly.  On  the  other  hand,  the  amount  of  reflection 
from  the  back  surface  of  a  particle  of  the  light  ordinarily  trans- 
mitted will  be  greater,  the  less  the  thickness  of  the  particle. 
Consequently,  with  fine  particles  there  will  be  a  more  or  less 
complete  reversal  of  the  usual  colors.  The  light  which  is  ordinar- 
ily reflected  selectively  will  be  transmitted  by  resonance,  while 
the  light  which  is  ordinarily  transmitted  is  scattered. 

This  is  known  to  be  true  for  indigo  and  for  gold.  Colloidal 
indigo  solutions  transmit  red  light  and  not  blue.  Massive  gold 
reflects  yellow  when  compact  and  brown  to  black  when  porous. 
Particles  which  do  not  resonate,  are  yellow  or  brown  by  reflected 
light  and  transmit  blue  light.  Massive  gold  is  red  by  multiple 
reflection  and  thin  films  are  green  by  transmitted  light.1  Col- 
loidal solutions  when  containing  very  fine  particles  reflect  green 
and  transmit  red.  In  the  red  gold  solutions  the  particles  are 
green,  while  they  are  yellow  to  brown  in  the  blue  solutions.  The 
violet  solutions  are  a  mixture  of  red  and  blue  solutions.  A  thin 
film  of  gold  does  not  resonate  readily  and  consequently  transmits 
green  light.  If  heated,  it  becomes  granular  and  then  transmits 
purple  light.  Gelatine  films  containing  gold  may  be  red  by 
transmitted  light  when  moist  and  blue  when  dry.  The  contrac- 
tion due  to  drying  brings  the  particles  of  gold  sufficiently  near 
together  to  be  equivalent  to  reversible  agglomeration,  thereby 
destroying  the  resonance.  In  a  colloidal  gold  solution  a  single 
particle  of  a  given  size  might  resonate  and  transmit  red  light, 
while  a  cluster  of  finer  particles  might  not  resonate  and  would 
therefore  transmit  blue  light.  This  is  probably  the  explanation 
for  some  of  the  abnormal  results  observed. 

Silver  reflects  yellow  after  multiple  reflection,  while  with 
fewer  reflections  the  light  is  said  to  be  red.2  A  very  fine  colloidal 
solution  transmits  yellow  and  scatters  blue,  while  somewhat 
coarser  particles  transmit  red  and  are  blue  by  reflected  light. 
The  coarsest  colloidal  silver  does  not  resonate  and  consequently 
is  yellow  by  reflected  light,  transmitting  blue  light.  This  is 
exactly  what  should  happen  on  the  facts  as  stated.  The  pink 
color  in  a  given  sample  of  white  lead  was  shown  by 

1  BEILBY:  Proc.  Roy.  Soc.,  72,  226  (1903). 

2  REGNAULT-BETTON:  Elements  of  Chemistry,  366  (1874). 


PROPERTIES  OF  COLLOIDAL  SOLUTIONS  205 

Baker1  to  be  due  to  silver,  which  was  probably  present  as  col- 
loidal silver.  The  colors  of  Carey  Lea's  photohalides  are  known 
to  be  due  to  colloidal  silver2  and  the  difference  in  the  color  of 
photographic  plates3  with  different  developers  is  due  to  the 
degree  of  fineness  of  the  precipitated  silver. 

Colloidal  platinum4  has  been  obtained  blue,  green,  yellow,  red, 
brown  and  black;  but  there  are  no  data  which  will  enable  us  to 
discuss  these  colors  satisfactorily. 

Owing  to  differences  in  the  variation  of  the  absorption  with 
varying  thickness  of  the  absorbing  agent,  a  substance  may  be 
one  color  in  thin  films  and  another  in  thicker  masses.  Thin 
layers  of  a  cyanine  solution  appear  blue  and  thick  layers  red,5 
and  the  same  is  true  of  cobalt  glass.  Potassium  ferricyanide  is 
a  brown-red  in  coarse  crystals  and  yellow  when  powdered. 
Andersen6  has  shown  that  thin  plates  of  hematite  (0.1/x)  are 
yellow  by  transmitted  light,  the  color  varying  with  increasing 
thickness  through  reddish  brown  to  deep  brown  red  or  blood  red. 
An  anhydrous,  yellow  ferric  oxide  is  said  to  be  obtained  by  the 
oxidation  of  ferric  sulphide7  and  we  get  yellow  bricks  on  burning 
a  highly  calcareous  clay  even  though  the  iron  content  is  higher 
than  that  which  would  give  a  red  clay  if  the  lime  were  not  pres- 
ent.8 It  seems  probable  that  the  ferric  oxide  is  present  in  such 
a  fine  state  of  subdivision  that  it  is  yellow  and  not  red.  Mott9 
has  prepared  both  red  and  yellow  ferric  oxide  by  volatilization 
of  iron  in  the  electric  arc,  the  yellow  being  the  finer  powder. 
Addition  of  a  liquid  having  approximately  the  same  index  of 
refraction  as  yellow  ferrocyanide  or  yellow  rouge  should  make 

1  Phil.  Mag.  (4)  37,  344  U869). 

2  CAREY  LEA:  Am.  Jour.  Sci.  (3)  33,  349,  489  (1887). 
SLUPPO-CRAMER:  Zeit.  Kolloidchemie,  7,  99  (1910);  8,  240  (1911);  CHAP- 
MAN JONES:  Phot.  Jour.,  51,  159  (1911). 

4  GARBOWSKI:  Ber.  deutsch.  chem.  Ges.,  36,  216  (1903);  GUTBIER:  Zeit. 
Kolloidchemie,  5,  50  ^1909);  WOHLER  and  SPENGEL:  Ibid.,  7,  243  (1910); 
BREDIG:  Zeit.  phys.  Chem.,  31,  258  (1899) 

6  WOOD:  Physical  Optics,  438  (1911). 

6  Am.  Jour.  Sci.  (4)  40,  370  (1915). 

7  DIAMOND:  Jour.  Soc.  Chem.  Ind.,  37,  451  R  (1918). 

"KEANE:  Jour.  Phys.  Chem.,  20,  734  (1916);  SCHEETZ:  Ibid.,  21,  570 
(1917). 

9  Trans.  Am.  Electrochem.  Soc.,  34,  292  (1918). 


206  APPLIED  COLLOID  CHEMISTRY 

the  powder  more  transparent  and  redder.  Experiments  by 
Schenck1  on  the  precipitation  of  hydrous  copper  and  aluminum 
oxides  make  it  seem  probable  that  cupric  oxide  is  blue  in  thin 
films  and  not  black.  Mott  obtained  a  brilliant  blue  by  volatiliz- 
ing a  mixture  of  equal  parts  of  manganese  and  copper. 

ELECTRICAL  PROPERTIES 

If  a  particle  suspended  in  a  liquid  adsorbs  a  free  ion,  the 
charged  particle  will  then  behave  to  some  extent  like  an  ion  and 
will  tend  to  move  under  electrical  stress  toward  the  cathode  if 
the  adsorbed  ion  is  positive  and  toward  the  anode  if  the  adsorbed 
ion  is  negative.2  Reference  has  been  made  to  the  fact  that 
silver  bromide  particles  move  to  the  cathode  when  stabilized  by 
silver  ions  and  to  the  anode  when  stabilized  by  bromine  ions.  By 
following  the  electrolysis  under  the  microscope  it  is  possible  to 
determine  the  rate  of  movement  of  a  suspended  particle  for  a 
given  difference  of  electrical  potential.  The  rate  of  migration 
of  colloidal  particles  is  of  the  same  order  as  that  of  ions.3  Thus 
colloidal  silver  moves  with  a  velocity  of  2.0-3.8/i/sec.  under  a 
potential  drop  of  one  volt  per  centimeter,  and  colloidal  gold  at 
about  4.0^/sec.,  while  the  chlorine  ion  gives  a  value  of  G.S/x/sec. 

Billitzer4  has  attempted  to  determine  absolute  potential  differ- 
ences by  finding  solutions  in  which  a  given  colloidal  metal  did 
not  move  under  electrical  stress.  While  the  method  is  good  in 
principle,  one  cannot  reason  from  the  adsorption  of  a  finely 
divided  metal  to  the  adsorption  of  massive  metal.  It  seems  pos- 
sible that  definite  results  might  be  obtained  if  the  problem  were 
attacked  with  a  better  knowledge  of  colloid  chemistry.  Since 
the  colloidal  particle  moves  in  one  direction  with  reference  to  the 
water,  the  water  moves  in  the  opposite  direction  relatively  to  the 
particle.  This  can  be  shown  if  we  keep  the  colloidal  particles 
stationary  in  the  form  of  a  diaphragm. 

The  electrical  transfer  of  particles  through  the  liquid  is  called 
cataphoresis  and  the  transfer  of  liquid  through  a  stationary  dia- 
phragm is  called  electrical  endosmose.  It  was  first  observed  by 

Mour.  Phys.  Chem.,  23,  284  (1919). 

2  See  LINDER  and  PICTON:  Jour.  Chem.  Soc.,  71,  568  (1897). 

3ZsiGMONDY:  The  Chemistry  of  Colloids,  46  (1917). 

4  Drude's  Ann.,  11,  902,  927  (1903). 


PROPERTIES  OF  COLLOIDAL  SOLUTIONS  207 

Reuss  at  Moscow  in  1808.  Working  with  so-called  pure  water 
it  is  found  that  water  passes  to  the  cathode  when  the  porous 
diaphragm  is  made  of  glass,  shellac,  earthenware,  carborundum, 
asbestos,  wool,  cotton,  platinum,  silver,  arsenic  sulphide,  or 
sulphur;  and  to  the  anode  when  the  diaphragm  is  chromic 
chloride,  cobalt  oxide,  zinc  oxide,  or  barium  carbonate.1  When 
turpentine  was  substituted  for  water,  it  moved  to  the  anode 
with  all  diaphragms  except  sulphur.  Coehn2  has  attempted  to 
account  for  this  by  postulating  empirically  that  when  two  non- 
miscible  substances  are  in  contact,  one  being  a  pure  liquid,  the 
substance  with  the  higher  dielectric  constant  is  charged  positively 
with  reference  to  the  one  with  the  lower  dielectric  constant.  This 
rule  applies  in  the  case  of  water  and  turpentine  where  the  dielec- 
tric constants  are  81  and  5  respectively,  but  it  is  by  no  means 
absolute  even  for  pure  liquids,3  and  breaks  down  completely 
when  applied  to  aqueous  solutions.  The  phenomenon  depends 
on  the  relative  adsorption  of  ions  and  Coehn' s  rule  is  a  useful 
first  approximation  in  certain  cases. 

The  first  quantitative  study  of  electrical  endosmose  with  solu- 
tions was  made  by  Wiedemann4  who  deduced  two  empirical 
generalizations : 

1.  The  mass  of  liquid  transported  through  a  porous  diaphragm 
in  a  given  time  is  directly  proportional  to  the  current.     For  a 
given  diaphragm  material  and  given  current,  the  amount  is 
independent  of  the  length  and  cross-section  of  the  diaphragm. 

2.  For  a  given  diaphragm  material  the  difference  in  hydrostatic 
pressure  maintained  between  the  two  sides  of  a  porous  diaphragm 
is  proportional  to  the  applied  potential  and  is  independent  of  the 
dimensions  of  the  diaphragm. 

•  Helmholtz5  considers  that  under  most  circumstances  liquids 
and  solids  become  charged  electrically  when  brought  into  con- 
tact, the  surface  of  the  solid  having  the  opposite  charge  from 
that  of  the  liquid  in  contact  with  it,  thus  giving  rise  to  a  so-called 

1  QUINCKE:  Pogg.  Ann.,  107,  1  (1859);  110,  38  (1860).     Actually  Quincke 
forced  water  through  the  diaphragms  and  noticed  the  charge. 

2  Wied.  Ann.,  64,  227  (1898);  Zeit.  Elektrochemie,  16,  586  (1910). 
3BRiGGs:  Jour.  Phys.  Chem.,  21,  204  (1917). 

«  Pogg.  Ann.,  87,  321  (1852);  99,  177  (1856). 
*  Wied.  Ann.,  7,  337  (1879). 


208  APPLIED  COLLOID  CHEMISTRY 

double  layer.  If  the  solid  cannot  move  because  it  is  a  capillary 
tube  or  a  diaphragm,  the  introduction  from  outside  of  a  potential 
gradient  will  tend  to  cause  a  displacement  of  the  electrically 
charged  layer  of  liquid.  If  the  liquid  is  not  a  perfect  insulator, 
the  displacement  results  in  a  continuous  flow  of  liquid  along  the 
surface  of  the  solid.1  Since  the  movement  of  a  suspended  and 
electrically  charged  particle  is  in  accord  with  Faraday's  law 
and  Kohlrausch's  law,  it  follows  that  the  movement  of  water 
relatively  to  the  diaphragm  will  not  be. 

Freundlich2  has  deduced  an  equation  for  the  amount  of  liquid 
V  transported  in  unit  time  through  a  porous  diaphragm: 


E  is  the  total  potential  drop  through  the  diaphragm,  D  is  the 
dielectric  constant  and  77  the  viscosity  coefficient  of  the  liquid, 
e  is  the  potential  of  the  Helmholtz  double  layer  at  the  solid- 
liquid  interface,  q  is  the  cross-section  and  I  the  thickness  of  the 
diaphragm.  If  7  is  the  specific  resistance  of  the  liquid,  then  the 
total  resistance,  R,  of  the  diaphragm  will  be  l/yq.  Substituting 
for  E  by  means  of  Ohm's  law  we  may  write  the  equation 


For  a  given  liquid  and  diaphragm  at  constant  temperature,  e, 
rj,  7  and  D  are  constant,  so  that  V  is  proportional  to  the  current 
and  the  term  for  the  diaphragm  disappears.  This  is  Wiede- 
mann's  first  empirical  generalization.  The  other  can  be  deduced 
by  introducing  an  expression  for  the  hydrostatic  pressure. 

Since  the  charge  on  the  diaphragm  depends  on  the  nature  of 
the  ion  adsorbed,  it  follows  that  an  aqueous  solution  may  flow  one- 
way or  the  other  through  a  given  diaphragm,  depending  on  the 
nature  of  the  dissolved  substance.  This  was  brought  out  clearly 
by  Parker3  and  was  emphasized  by  Perrin,4  who  found  a  reversal 
in  many  cases  when  the  solutions  were  alkaline  instead  of  acid. 

JLAMB:  Phil.  Mag.  (5)  25,  52  (1888). 

2  Kapillarchemie,  226  (1909). 

3  Johns  Hopkins  Dissertations,  31,  23  (1901);  BRIGGS:  Jour.  Phys.  Chem., 
21,  235  (1917). 

4  Jour.  Chim.  phys.,  2,  G01  (1904). 


PROPERTIES  OF  COLLOIDAL  SOLUTIONS  209 

Every  diaphragm  tends  to  become  charged  positively  in  an  acid 
solution  and  negatively  in  an  alkaline  solution.  Every  ion  of 
unlike  sign  tends  to  neutralize  the  charge  on  the  diaphragm. 
Perrin  believed  that  the  effect  of  other  ions  was  more  marked  the 
greater  the  valence;  but  this  is  true  only  in  so  far  as  the  ion  with 
the  higher  valence  is  adsorbed  more  strongly  than  the  ion  with  a 
lower  valence,1  in  other  words,  in  so  far  as  Schulze's  so-called 
law  holds.  In  some  cases  the  agreement  is  very  good.  Thus  in 
acid  solutions,  the  flow  through  the  diaphragm  to  the  anode 
increases  as  the  anion  changes  from  quadrivalent  ferrocyanide 
to  bivalent  sulphate  to  univalent  chloride,  because  the  adsorption 
of  the  anion  decreases  in  the  same  order  and  the  diaphragm  be- 
comes charged  more  positively.  In  alkaline  solution  the  flow 
to  the  cathode  increases  as  the  cation  changes  from  trivalent 
lanthanum  to  bivalent  barium  to  univalent  sodium.  On  the 
other  hand,  with  a  porous  cup  in  a  slightly  acidified  copper 
sulphate  solution  the  flow  is  to  the  cathode,  apparently  because 
copper  ions  are  not  adsorbed  to  any  great  extent,  while  the  ad- 
sorption of  sulphate  ions  exceeds  the  adsorption  of  hydrogen  ions 
because  the  concentration  of  the  first  is  so  much  greater  than  that 
of  the  second.  With  a  slightly  acidified  copper  nitrate  solution 
the  flow  is  to  the  anode,  because  the  nitrate  ions  are  not  adsorbed 
very  much  and  the  hydrogen  ions  determine  the  direction  of  the 
flow. 

Much  to  Perrin's  surprise  a  barium  carbonate  solution  acted 
in  one  case  as  though  it  were  an  acid.  This  is  undoubtedly 
because  of  the  preferential  adsorption  of  barium  ions  which 
charge  the  diaphragm  positively. 

In  electrolytic  reductions  of  organic  substances  in  aqueous 
alcoholic  solutions  using  a  porous  cup,  the  flow  of  liquid  to  the 
cathode  may  be  quite  annoying.  Owing  to  the  lower  dielectric 
constant  of  alcohol,  the  normal  tendency  of  the  liquid  to  go  to 
the  cathode  becomes  less,  but  the  increased  resistance  would 
increase  the  flow  and  nothing  is  known  in  regard  to  adsorption 
from  these  solutions.  Billitzer2  found  that  colloidal  platinum  is 
charged  positively  in  aqueous  alcohol  though  negatively  in  pure 
water. 

1  ELISSAFOFF:  Zeit.  phys.  Chem.,  79,  285  (1912). 

2  Zeit.  phys.  Chem.,  43,  312  (1903). 

14 


210  APPLIED  COLLOID  CHEMISTRY 

The  amount  of  liquid  transferred  by  electrical  endosmose 
increases  with  rising  temperature,  the  increase  being  nearly  pro- 
portional to  the  change  in  the  fluidity1  (the  reciprocal  of  the 
viscosity).  This  would  follow  from  Freundlich's  equation  if 
one  assumes  that  D  and  c  change  but  slightly  over  the  tem- 
perature range. 

The  commercial  applications  of  electrical  endosmose  have  not 
been  very  successful.  Count  Botho  Schwerin2  took  out  a 
number  of  patents  for  removing  water  from  peat.  Starting 
with  a  peat  mud  containing  about  90  percent  moisture,  he 
claimed  that  the  moisture  content  could  be  reduced  to  65  per- 
cent at  an  expenditure  of  about  15  kw.-hr.  per  cubic  meter  of 
water  removed,  the  best  potential  gradient  being  4-5  volts  per 
centimeter.  The  cost  of  drying  from  65  percent  moisture  to 
20  percent  moisture  was  so  great  that  the  process  was  not  a 
success.  It  is  reported  that  electrical  endosmose  has  been  used 
successfully  in  the  dye  industry  in  the  drying  of  pastes.  Elec- 
trical tanning  has  also  been  tried,  the  tanning  liquor  being  carried 
into  the  hides  by  electrical  endosmose.  This  seems  to  have 
failed  in  part  on  account  of  the  oxidation  of  the  tannin  at  the 
anode.  Cataphoresis  is  made  use  of  in  electrolytic  refining 
and  plating,  the  use  of  addition  agents  being  essential  in  some 
cases.3 

There  are  no  very  definite  results  as  to  the  effect  of  colloids 
on  the  conductance  of  solutions,4  but  it  is  evident  that  an  ad- 
sorbed ion  cannot  have  as  high  a  migration  velocity  as  an  un- 
adsorbed  ion,  and  that  consequently  the  presence  of  a  colloid 
must  decrease  the  conductance  to  some  extent,  depending  on  the 
amount  and  nature  of  the  ions  adsorbed.  If  a  clay  suspension 
is  stirred,5  the  conductance  may  increase  because  the  adsorbed 
ions  which  are  carried  down  when  the  suspension  settles  are 
brought  up  between  the  electrodes.  McBain6  found  that  certain 
solutions  of  sodium  palmitate  conducted  well,  although  the 

1  BRIGGS,  BENNETT  and  PIERSON:  Jour.  Phys.  Chem.,  22,  256  (1918). 

2  Zeit.  Elektrochemie,  9,  739  (1903). 

3  MULLER  and  BAHNTJE:  Zeit.  Elektrochemie,  12,  317  (1906). 

4  Wo.  OSTWALD:  Van  Bemmelen  Gedenkboek,  267  (1910). 
e  BLEININGER:  Trans.  Am.  Ceramic  Soc.,  15,  343  (1913). 

6  Jour.  Chem.  Soc.,  106,  965  (1914);  Jour.  Am.  Chem.  Soc.,  42,  426  (1920). 


PROPERTIES  OF  COLLOIDAL  SOLUTIONS  211 

freezing  points  of  the  solutions  were  only  enough  to  account  for 
the  sodium  ions.  He  assumes  that  the  palmitate  ions  polymerize 
to  large  hydrated  units  which  he  calls  micelles  and  which  he 
considers  colloidal  and  to  which  he  applies  Stokes'  law  although 
he  does  not  state  specifically  why  or  how  the  polymerized  ions 
separate  as  a  second  phase.  It  is  probable  that  we  have  hydro- 
lysis to  caustic  soda  and  colloidal  palmitic  acid,  the  latter1  then 
adsorbing  hydroxyl  ions,  just  as  we  know  that  silicic  acid  adsorbs 
hydroxyl  ions  and  is  peptized  by  them.  This  suggests  an  inter- 
esting theoretical  possibility.  Suppose  we  have  two  sets  of  finely 
divided  particles  neither  of  which  adsorbs  the  other  appreciably 
and  let  us  also  suppose  that  one  set  of  particles  adsorbs  a  given 
cation  very  strongly  while  the  other  set  of  particles  adsorbs  a 
given  anion  very  strongly.  If  we  take  a  mixture  of  these  two 
sets  of  particles  and  add  a  small  amount  of  the  salt  of  the  given 
base  and  the  given  anion,  we  shall  have  a  colloidal  solution  which 
will  conduct  electricity  very  well  but  which  will  contain  no  free 
ions  to  speak  of  because,  by  definition,  the  cations  have  been 
practically  completely  adsorbed  by  one  set  of  particles  and  the 
anions  by  the  other  set  of  particles.  It  is  not  known  whether 
these  limiting  conditions  can  all  be  fulfilled  simultaneously. 
A  possible  case  would  be  the  mixing  of  dilute  solutions  of  silver 
sulphate  and  lead  chloride.  The  lead  sulphate  would  adsorb 
sulphate  or  lead  ions  strongly  and  the  silver  chloride  would  adsorb 
chloride  or  silver  ions  strongly.  Nobody  knows  what  each  would 
do  to  the  other. 

While  an  adsorbed  ion  may  increase  the  conductance  of  a 
liquid,  it  will  behave  quite  abnormally  as  regards  electrometric 
measurements.  An  adsorbed  chlorine  ion  will  not  give  a  test 
with  silver  nitrate  and  consequently  will  have  only  a  negligible 
effect  on  a  chlorine  electrode.  It  is  not  probable  that  an  ad- 
sorbed hydrogen  or  hydroxyl  ion  behaves  electrometrically  like 
a  free  hydrogen  or  hydroxyl  ion.  This  possible  error  has  been 
overlooked  in  many  cases  where  the  hydrogen  electrode  has  been 
used.  Another  source  of  error  in  electrometric  measurements 
with  colloidal  solutions  is  to  neglect  the  fact  that  we  are  not 
dealing  with  true  solutions.  If  we  measure  a  copper  electrode 
in  a  solution  of  alkaline  copper  tartrate  we  find  that  the  concen- 

1  There  may  also  be  adsorption  by  colloidal  sodium  palmitate. 


212  APPLIED  COLLOID  CHEMISTRY 

tration  of  copper  ions  is  extremely  low.  That  is  all  that  the 
measurement  tells  us.  If  the  copper  is  in  true  solution,  it  must 
be  present  as  a  complex  salt,  as  is  unquestionably  the  case  with 
potassium  silver  cyanide.  If  we  have  peptized  copper  oxide  or 
hydroxide  present,  there  is  no  need  to  assume  the  existence  of  a 
complex  copper  salt  at  all.  The  true  interpretation  of  the  phe- 
nomena cannot  be  made  until  we  know  whether  a  so-called  alka- 
line copper  tartrate  solution  is  a  true  solution  or  not.  We 
cannot  answer  this  question  at  present.  On  the  other  hand,  it 
seems  practically  certain  that  the  effect  of  sugars  in  preventing 
the  precipitation  of  the  heavy  metal  hydroxides  by  alkalies  is 
due  to  the  formation  of  colloidal  solutions.  Consequently  the 
conclusions  drawn  from  electrometric  measurements  of  such 
solutions  are  necessarily  false. 

STABILITY  OF  COLLOIDAL  SOLUTIONS 

1.  Sols  stabilized  by  Ions. — Three  classes  of  colloidal  solutions 
have  been  distinguished,  in  which  the  stabilization  is  due:  to 
adsorbed  liquid;  to  an  adsorbed  undissociated  substance  which 
may  be  in  true  solution  or  may  be  itself  in  colloidal  solution; 
and  to  an  adsorbed  ion.  The  last  case  is  the  simplest  theoreti- 
cally and  the  most  varied  experimentally.  So  long  as  the  parti- 
cles are  all  charged  positively  or  all  charged  negatively,  they  will 
tend  to  repel  each  other  and  will  not  coalesce  unless  the  concen- 
tration is  too  high.  If  the  charge  is  neutralized  or  counter- 
balanced in  any  way,  the  particles  will  agglomerate1  unless  some 
other  factor  comes  into  play.  Such  colloidal  solutions  will 
therefore  be  very  sensitive  to  electrolytes  having  a  readily  ad- 
sorbed ion  of  the  opposite  charge  from  that  which  stabilizes  the 
colloidal  solution.  Colloidal  solutions  which  are  peptized  by 
the  liquid  will  be  relatively  insensitive  to  electrolytes  because  these 
will  act  only  in  so  far  as  they  decrease  the  peptizing  action  of  the 
liquid.  The  second  case  is  intermediate  with  reference  to  the 
other  two  and  its  properties  will  vary  between  those  of  the  other 
two  types. 

We  will  consider  first  the  case  in  which  the  colloid  is  stabilized 

1  HARDY:  Zeit.  phys.  Chem.,  33,  285  (1900);  BURTON:  Phil.  Mag.  (6)  12, 
472  (1906);  17,  583  (1909). 


PROPERTIES  OF  COLLOIDAL  SOLUTIONS  213 

chiefly  or  entirely  by  the  presence  of  an  adsorbed  ion.  Such 
solutions  precipitate  completely  at  the  iso-electric  point,  as  it  is 
called,  where  the  electrical  charge  is  neutralized  or  counter- 
balanced. If  a  suspension  is  stabilized  by  the  preferential 
adsorption  of  hydrogen  ion  from  hydrochloric  acid  solution,  the 
solution  contains  free  hydrogen  ions,  free  chlorine  ions,  and  the 
adsorbed  hydrogen  ions  which  make  the  suspension  behave  like 
a  cation  though  with  a  different  migration  velocity  from  that 
of  hydrogen.  If  the  suspension  is  made  to  adsorb  an  anion  in 
an  amount  equivalent  to  the  hydrogen  ion  adsorbed,  the  sus- 
pended particles  will  be  electrically  neutral.  This  can  be  done 
by  adding  an  electrolyte  with  a  readily  adsorbed  anion.  Since 
this  is  a  matter  of  selective  adsorption,  the  concentration  of  the 
added  anion  necessary  to  cause  an  adsorption  equivalent  to 
the  hydrogen  adsorption  will  vary  with  each  anion.  To  put  the 
matter  more  generally,  the  amount  of  an  electrolyte  necessary 
to  precipitate  a  colloidal  solution  will  vary  with  the  nature  of  the 
cation,  the  anion,  and  the  dispersed  phase.  While  it  is  generally 
true  that  an  ion  of  higher  valence  will  be  adsorbed  more  strongly 
than  one  of  lower  valence,  this  so-called  law  of  Schulze1  is  only  a 
first  approximation  and  should  be  considered  only  as  a  guide. 

That  some  univalent  ions  are  adsorbed  more  strongly  by 
some  substances  than  some  bivalent  or  trivalent  ions  is  shown 
clearly  in  data  by  Ode*n2  on  colloidal  sulphur,  given  in  Table 
XLIII.  In  the  second  column  are  the  liminal  concentrations 
necessary  to  coagulate  the  sulphur,  given  in  gram  ions  per  liter 
of  the  cations;  in  the  third  column  are  given  the  reciprocals  of 
these  values,  the  so-called  ionic  precipitating  power. 

Under  the  conditions  of  OdeVs  experiments,  sulphur  is  a 
negative  colloid  and  the  precipitation  is  therefore  due  to  an 
adsorption  of  cations.  The  first  thing  to  be  noticed  is  that 
hydrogen  ion  is  not  adsorbed  strongly  by  sulphur,  the  precipitat- 
ing power  of  hydrochloric  acid  being  much  less  than  that  of 
lithium,  ammonium,  sodium,  potassium,  rubidium,  or  caesium 
chloride.  Instead  of  these  univalent  cations  precipitating  at  the 
same  concentration,  the  required  concentration  of  lithium  chlo- 
ride is  in  round  numbers  one  hundred  times  that  of  caesium 

1  Jour,  prakt.  Chem.  (2)  25,  431  (1881);  27,  320  (1884). 

2  Der  kolloide  Schwefel,  156  (1912). 


214  APPLIED  COLLOID  CHEMISTRY 

TABLE  XLIII. — COAGULATION  OF  SULPHUR  AT  18°-20° 


Salt 

Liminal  value  gram- 
ions  cations  per  liter 

Ionic  precipitating 
power  of  cation 

HC1                              

6.0 

0.16 

LiCl 

0  913 

1  1 

NH4C1 

0  435 

2  3 

(NH4)2SO4       

0.600 

1.7 

NH4NO3                         

0.506 

2.0 

NaCl 

0  153 

6.1 

Na2SO4  

0.176 

5.7 

NaNO3                

0.163 

6.1 

KC1                                               .  .    . 

0  021 

47.5 

K2SO4 

0  025 

39  7 

KNO3  
RbCl                      

0.022 
0.016 

45.5 
63.0 

CsCl 

0  009 

108  0 

MgSO4  

0.0093 

107.5 

Mg(NOs)a               

0.0080 

125.0 

CaCl2 

0  0041 

245  0 

Ca(NO3)2  

0.0040 

247.0 

Sr(N03)2  
BaCl2 

0.0025 
0  0021 

385.0 
475.0 

Ba(NO3)2  

0.0022 

461.0 

ZnSO4  
Cd(NO3)2 

0.0756 
0  0493 

13.2 
20.3 

A1C13 

0  0044 

227  0 

CuS04  
Mn(NO3)2  

Ni(NO3)2 

0.0098 
0.0096 
0  0446 

102.0 
105.0 
22.4 

UO2(NO3)2  

0.0137 

73.0 

chloride.  The  liminal  values  for  barium  and  strontium  are 
nearly  equal,  but  calcium  chloride  requires  a  distinctly  higher 
concentration.  If  we  take  the  different  bivalent  ions  the  values 
range  from  0.0756  for  zinc  to  0.0022  for  barium,  a  ratio  of  over 
thirty  to  one.  The  univalent  caesium  ion  has  a  greater  precipi- 
tating power  than  the  bivalent  zinc,  cadmium,  nickel,  and  uranyl 
ions;  and  about  the  same  precipitating  power  as  the  bivalent 
copper,  manganese,  and  magnesium  ions.  The  trivalent  alumi- 
num ion  has  about  the  same  precipitating  power  as  the  bivalent 
calcium  ion  and  distinctly  less  precipitating  power  than  bivalent 
strontium  and  barium  ions.  The  specific  nature  of  the  adsorp- 


PROPERTIES  OF  COLLOIDAL  SOLUTIONS  215 

tion  comes  out  extraordinarily  clearly  with  sulphur,  about  the 
only  orthodox  thing  being  that  nitrate,  chloride,  and  sulphate 
behave  practically  alike,  though'  even  here  Ode*n  considers  that 
sulphate  has  a  slight  protecting  action.  This  specific  nature 
appears  more  clearly  perhaps  if  we  arrange  the  cations  in  order, 
the  one  with  the  greatest  precipitating  power  coming  first: 
Ba,  Sr>Ca,  Al>Mg,  Cs,  Mn,  Cu>U02>Rb>K>Ni,  Cd, 
Zn>Na>NH4>Li>H. 

Sulphur  is  admittedly  an  extreme  case,  but  Freundlich1  gives 
data  for  colloidal  platinum  from  which  I  deduce  the  order:  Al, 
Pb>Ba,  U02>Ag>K,  Na.  Bivalent  lead  has  practically  the 
same  precipitating  power  as  trivalent  aluminum.  Univalent 
silver  is  nearer  to  bivalent  uranyl  and  barium  than  to  univalent 
potassium  and  sodium.  If  more  cations  had  been  studied  we 
should  very  likely  have  got  more  distinct  evidence  of  specific 
action.  As  it  is,  it  takes  130  millimols  NaOH  per  liter  to  coagu- 
late the  platinum  and  only  2.5  millimols  NaCl.  The  change 
from  chloride  to  hydroxide  has  a  more  marked  effect  than  the 
change  from  sodium  to  barium.  It  seems  very  probable  that 
barium  hydroxide  would  have  no  greater  precipitating  power  than 
sodium  chloride.  From  Pappada's  experiments  with  colloidal 
silver2  I  deduce  the  following  order  of  adsorption:  Al>Ba,  Sr, 
Ca>H>Cs>Rb>K>Na>Li.  From  these  data  Pappada  con- 
cludes that  the  migration  velocity  is  the  determining  factor 
with  the  univalent  cations;  but  this  cannot  be  true.  The  differ- 
ence between  aluminum  and  hydrogen  is  not  very  great,  one 
drop  of  M/10  HC1  producing  a  coagulation  and  one  drop  M/20 
A1C13.  In  tenth-normal  solutions  potassium  iodide,  nitrate, 
and  sulphate  produce  no  coagulation.  The  reason  given  by  the 
author  is  that  these  anions  react  with  the  colloidal  silver.  In 
normal  solutions  the  iodides,  nitrates,  and  sulphates  are  said  to 
precipitate  at  the  same  concentrations  as  the  corresponding 
chlorides  and  bromides.  The  effect  of  concentration  is  a  little 
obscure  in  other  respects,  since  5  or  6  drops  of  normal  KC1  pre- 
cipitate 2  cc  0.06  percent  Ag,  whereas  it  takes  only  30  drops 
N/10  KC1  to  produce  precipitation.  The  essential  thing  is  that 
the  different  univalent  cations  have  different  liminal  values;  the 

1  Kapillarchemie,  352  (1909). 

2  PAPPADA:  Gazz.  chim.  ital.,  42  I,  263  (1912). 


216 


APPLIED  COLLOID  CHEMISTRY 


difference  between  hydrogen  and  lithium  is  greater  than  that 
between  hydrogen  and  aluminum.  Experiments  might  well 
have  been  made  with  barium  hydroxide  and  with  the  sodium 
salts  of  organic  acids.  Freundlich  and  Leonhardt1  find  that  with 
the  negative  colloid,  MoO2.H2O,  caesium  is  two  hundred  times 
as  effective  as  lithium,  and  barium  four  times  as  effective  as 
calcium. 

TABLE  XLIV. — COAGULATION  OF  MASTIC 


Salt 

Liminal  value  gram 
atoms 

Ionic  precipitating 
power  of  cation 

NaCl                     

1.0 

1 

AgNO3 

0  125 

8 

HgNO3  

0.00125 

800 

HC1  
CaCl2 

0.010 
0  025 

100 

40 

BaCl2 

0  025 

40 

ZnSO4  
A12(SO4)3                                          .    . 

0.050 
0  0004 

20 
2500 

A12(NO3)3 

0  0004 

2500 

FeCl3  

0.0003 

3300 

From  experiments  on  mastic2  we  get  the  data  given  in  Table 
XLIV.  If  we  consider  the  mercury  in  mercurous  nitrate  as  a 
univalent  ion,  it- is  very  much  out  of  place,  precipitating  at  much 
lower  concentrations  than  the  barium,  calcium  and  zinc  salts. 
Of  course  the  formula  should  be  written  Hg2(NO3)2  with  Hg2  as 
a  bivalent  ion.3  In  this  case  the  precipitating  power  becomes 
1600  instead  of  800,  which  puts  it  up  much  nearer  the  trivalent 
cations  than  the  bivalent  ones.  The  order  of  cations  is:  Fe, 
Al>Hg2>H>Ba,  Ca>Zn>Ag>Na.  Only  three  anions  are 
given  in  the  table,  so  it  is  impossible  to  tell  what  effect  the  anions 
have.  A  good  many  experiments  have  been  made  on  mastic 
with  different  acids  but  the  degree  of  electrolytic  dissociation 
varies  so  as  to  make  these  results  inconclusive.  With  Prussian 
blue  Pappada4  found  the  order  of  the  cations  to  be :  Fe,  Al,  Cr  > 

1  Kolloidchemische  Beihefte,  7,  172  (1915). 

2  FREUNDLICH:  Kapillarchemie,  367  U909). 

3  OGG:  Zeit.  phys.  Chem.,  27,  285  (1898). 

4  Zeit.  Kolloidchemie,  6,  83  (1911). 


PROPERTIES  OF  COLLOIDAL  SOLUTIONS  217 

Ba,  Cd  >  Sr,  Ca  >  H  >  Cs  >  Rb  >  K  >  Na  >Li.  Sulphates,  nitrates, 
chlorides,  bromides,  and  iodides  all  behaved  alike.  Practically 
the  same  order  of  the  cations  was  obtained  for  copper  ferrocyan- 
ide.1  In  the  cases  studied  by  Pappada  the  specific  adsorption 
appears  to  play  a  very  small  part.  The  data  for  arsenic  sulphide,2 
however,  give  variety  enough.  The  order  of  cations  is:  Ce,  In, 
benzidine,  Al>new  fuchsine,  crystal  violet  >  quinine  >  morphine, 
UO2,  Sr,  Ca>Be,  Zn,  Ba  >  Hg  >  p-chloraniline,  toluidine  >  anil- 
ine >  strychnine  >  guanidine  >  H  >  K  >  Na  >Li.  The  organic 
cations  come  in  where  they  please  and  play  havoc  with  any  rule 
as  to  valency.  The  chlorides  and  nitrates  give  practically  the 
same  values  and  the  sulphates  are  not  far  out  of  line,  though  it 
seems  probable  that  the  restraining  power  of  sulphate  is  rather 
greater  than  that  of  chloride  or  nitrate.  The  liminal  values  in 
gram  molecules  of  the  cation  per  liter  are  0.0056,  0.0066,  0.0086, 
0.110  and  0.240  for  potassium  nitrate,  sulphate,  formate,  acetate, 
and  citrate,  from  which  one  can  deduce  that  the  order  of  adsorp- 
tion of  anion  is :  citrate,  acetate,  formate,  sulphate,  nitrate,  chloride. 
It  is  a  great  pity  that  Freundlich  did  not  try  other  combinations, 
such  as  barium  acetate  for  instance.  Addition  of  amyl  alcohol 
'or  of  phenol  makes  the  arsenic  sol  more  susceptible  to  univalent 
and  trivalent  cations3  and  less  sensitive  to  bivalent  and  quadri- 
valent cations,  but  we  do  not  know  why. 

From  the  experiments  on  hydrous  ferric  oxide,4  the  order  of 
adsorption  of  the  precipitating  anions  appears  to  be  Cr2C>7>SO4 
>  OH  >  salicylate  >  benzoate  > formate  >  Cl  > No3  >  Br  >  I,  while 
the  order  for  the  cations  is:  H>Ba>Mg>Tl,  Na>K.  The 
univalent  ions  do  not  all  behave  alike  and  neither  do  the 
bivalent  ones;  but  the  upholders  of  Schulze's  law  can  comfort 
themselves  with  the  fact  tfrat  the  two  sets  do  not  overlap  except 
in  the  case  of  hydrogen.  There  is  no  such  comfort  in  the  case 
of  albumin.  The  probable  order  of  adsorption  of  anions,  so  far 
as  known,  is:  sulphocyanate,  iodide > chlorate > nitrate > chloride 

1  PAPPADA:  Zeit.  Kolloidchemie,  9,  136  (1911). 

2  FREUNDLICH:  Kapillarchemie,  351  (1909);  FREDNDLICH  and  SCHUCHT: 
Zeit.  phys.  Chem.,  80,  564  (1912). 

3  KRUYT  and  DUIN:  Kolloidchemische  Beihefte,  5,  269  (1913). 

4  FREUNDLICH:    Kapillarchemie,   352,   358   (1909);  ZSIGMONDY:  .Kolloid- 
chemie, 181  (1912);  PAPPADA:  Zeit.  Kolloidchemie,  9,  233  (1911). 


218  APPLIED  COLLOID  CHEMISTRY 

>  acetate  >  phosphate  >  sulphate  >  tartrate,    the    sulphocyanate 
ion  being  adsorbed  the  most  and  the  tartrate  ion  the  least.     Here 
there  is  nothing  even  to  suggest  Schulze's  law  and  the  firm  belief 
which  most  people  have  in  Schulze's  law  is  probably  one  reason 
for  the  marked  failure  to  account  satisfactorily  for  the  phenom- 
ena with  albumin.     With  the  cations  albumin  appears  to  be 
fairly  orthodox  for  the  order  of  adsorption  appears  to  be:  Th, 
UO2>Cu,  Zn>Ca>Mg>Li>K,  Na>NH4,  though  even  here 
the  lithium  stands  higher  in  the  series  than  it  has  been  found 
with  other  substances. 

Weiser  and  Sherrick1  have  measured  the  adsorption  of  ions  by 
barium  sulphate  and  find  that  the  order  is :  f errocyanide  >  nitrate 

>  nitrite  >  chlorate  >  permanganate  >  f erricyanide  >  chloride  > 
bromide  >  cyanide  >  sulphocyanate  >  iodide,    the    f  errocyanide 
ion  being  adsorbed  the  most  and  the  iodide  ion  the  least.     The 
above  order  is  obtained  when  the  concentrations  are  given  in 
gram  equivalents.     If  they  are  taken  as  gram  molecules,  f  erro- 
cyanide moves  down  between  the  chlorate  and  permanganate, 
and  the  ferricyanide  between  the  chloride  and  bromide.     On 
either  method  of  tabulation  the  trivalent  ferricyanide  ion  is 
adsorbed  very  much  less  than  some  of  the  univalent  ions.     The 
difference  for  different  univalent  ions  is  very  great,  100  mols  of 
barium  sulphate  taking  up  about  8.5  mols  of  nitrate  as  against 
about  0.06  of  iodide. 

Weiser  and  Middleton2  have  determined  the  order  of  precipi- 
tation of  colloidal  ferric  oxide  by  different  anions  and  obtain 
results  which  agree  with  the  previous  data  in  so  far  as  they  deal 
with  the  same  electrolytes,  the  order  being:  dichromate> tartrate 

>  sulphate  >  citrate  >oxalate>iodate>  phosphate.     They  have 
also  determined  the  amount  of  adsorption  when  ferric  oxide  is 
precipitated  in  presence  of  a  definite  excess  of  salt.     This  order 
is  quite  different,  being :  phosphate  >  citrate  >  tartrate  >oxalate  > 
sulphate  >iodate>dichromate.     The  reason  for  the  discrepancy 
is  apparently  that  there  is  an  adsorption  of  the  electrically 
neutral  salt  in  addition  to  the  ion  adsorption.     If  this  is  selective, 
it  may  change  the  order  to  any  extent  and  apparently  does  change 
it  very  much  in  this  case.     The  difficulty  with  this  is  that  it 

1  Jour.  Phys.  Chem.,  23,  205  (1919). 

2  Ibid.,  24,  30  (1920). 


PROPERTIES  OF  COLLOIDAL  SOLUTIONS  219 

leaves  us  with  no  independent  proof  that  the  order  of  precipita- 
tion is  really  the  same  as  the  order  of  adsorption  of  the  ions. 
This  confirmation  will  undoubtedly  be  furnished  some  day;  but, 
until  it  is,  the  connection  between  adsorption  and  precipitation 
can  only  be  considered  as  extremely  probable  but  as  not  proved. 
What  we  actually  measure  is  the  adsorption  necessary  to  cause 
precipitation  plus  the  adsorption  by  the  precipitated  substance. 
This  latter  item  can  theoretically  be  made  as  small  as  one  pleases 
by  adding  only  enough  electrolyte  to  precipitate  an  infinitely 
small  amount  of  the  colloid;  but  the  errors  of  analysis  then 
become  very  large. 

While  there  is  unquestionably  a  tendency  for  ions  of  a  higher 
valence  to  be  adsorbed  more  strongly  than  ions  of  a  lower  va- 
lence, the  experiments  which  have  been  cited  show  that  there 
are  many  exceptions1  and  that  the  fundamental  rule  is  that  the 
adsorption  is  specific  both  as  regards  the  adsorbing  substance 
and  the  ion  adsorbed. 

Albumin  is  a  specially  interesting  case  because  it  is  peptized 
readily  by  cations  or  anions.  When  it  is  negatively  charged, 
as  in  a  slightly  alkaline  solution,  a  strongly  adsorbed  anion  will 
make  it  more  negative  and  more  stable.  Consequently  precipi- 
tation by  a  sodium  salt  will  be  more  effective  the  less  readily  the 
anion  is  adsorbed.  On  the  other  hand,  in  acid  solutions  the 
sodium  salt  with  the  most  strongly  adsorbed  anion  will  be  the 
most  effective  in  causing  precipitation.  Negatively  charged 
albumin  is  precipitated  readily  by  sodium  chloride  and  not  at  all 
by  sodium  iodide,  while  positively  charged  albumin  is  precipi- 
tated by  sodium  iodide  much  more  readily  than  by  sodium  chlo- 
ride. Potassium  iodide2  has  more  effect  than  potassium  chloride 
with  Rhodamine  B  while  the  reverse  is  true  with  wool  violet  S. 

Caseine  has  not  been  studied  so  intelligently  as  albumin  and  the 
results  are  consequently  less  clear.  Caseine  is  insoluble  in  water 
but  reddens  litmus  paper  and  sets  free  carbon  dioxide  from 
carbonates  and  bicarbonates.  It  is  therefore  usually  considered 
an  acid  whereas  the  phenomenon  is  really  due  to  a  marked  ad- 
sorption of  the  base.  Owing  to  adsorption,  indicators  are  usually 
unreliable  in  protein  solutions  as  a  means  of  determining  hy- 

JSee  also  Mines:  Kolloidchemische  Beihefte,  3,  212  (1912). 
2  TRAUBE:  Kolloidchemische  Beihefte,  3,  241  (1912). 


220  APPLIED  COLLOID  CHEMISTRY 

drogen  ion  concentration.  Like  albumin,  caseine  is  carried  into 
apparent  solution  by  acids,  alkalies,  alkaline  earths,  and  some  salts. 
It  is  also  peptized  by  caffeine,  strychnine,  etc.1  The  so-called 
solutions  are  opalescent  and  the  caseine  can  be  removed  by  ultra- 
filtration,  showing  that  it  is  not  in  true  solution.  As  one  might 
expect,  the  peptizing  action  of  the  alkaline  earths  is  less  marked 
than  that  of  the  alkalies,  because  the  adsorption  of  the  bivalent 
cations  cuts  down  the  peptizing  action  of  the  hydroxyl.  When 
peptized  by  hydroxyl,  caseine  is  precipitated  by  solutions  of 
calcium  chloride,  barium  chloride,  magnesium  sulphate,  zinc 
sulphate,  copper  sulphate,  and  alum.  In  these  cases  it  seems 
probable  that  the  negative  charge  is  neutralized  by  the  readily 
adsorbed  cations.  On  the  other  hand,  when  peptized  in  acict 
solution,  salts  cause  precipitation  in  the  following  order:  ZnCl2 
>KBr>CuCl2>MnCl2.  There  is  no  apparent  reason  for  this 
and  there  must  be  experimental  error  somewhere.  In  fact  the 
whole  behavior  of  caseine  in  presence  of  salts  needs  careful 
revision  because  the  present  data  are  quite  unsatisfactory. 

Colloidal  silicic  acid  behaves  like  caseine  in  one  respect,  that 
it  adsorbs  lime  from  suspended  calcium  carbonate.  In  accord- 
ance with  this  is  the  fact  that  silicic  acid,  though  peptized  by 
caustic  soda  or  ammonia,  is  precipitated  by  barium  hydroxide 
because  of  the  marked  adsorption  of  the  alkaline  earth  cation. 
It  is  also  precipitated  by  alum  and  by  basic  dyes,  which  is  as  it 
should  be. 

With  colloids  stabilized  by  an  electrical  charge,  we  find  that 
the  amount  of  any  salt  necessary,  to  produce  precipitation 
varies  with  the  way  in  which  it  is  added,  more  being  necessary 
if  the  salt  solution  is  added  slowly.  The  explanation  of  this 
seems  to  be  that  the  first  addition  of  salt  causes  a  slight  agglom- 
eration which  is  not  enough  to  cause  precipitation,  but  which 
cuts  down  the  adsorbing  power  of  the  colloid,  so  that  more  salt  is 
necessary  to  cause  the  same  adsorption.  It  is  also  true  that  a 
change  in  the  adsorbing  power  of  a  colloidal  substance  will 
affect  its  stability.  If  we  have  anhydrous  ferric  oxide  on  the 
one  hand  and  hydrous  ferric  oxide  on  the  other,  without  specify- 
ing at  present  what  we  mean  by  hydrous,  we  shall  expect  to 


:  Jour.  Physiology,  20,  399  (1910);  VAN  SLTKE  and  PUBLOW: 
Science  and  Practice  of  Cheese-making,  142  (1913). 


PROPERTIES  OF  COLLOIDAL  SOLUTIONS  221 

have  different  degrees  of  stability  in  the  two  cases  and  so  we  do. 
Two  colloidal  ferric  oxides  have  been  distinguished  and  we  speak 
of  colloidal  solutions  of  alumina  and  metalumina,  tungstic  acid 
and  metatungstic  acid,  stannic  and  metastannic  acid,  etc.,  where 
our  only  real  criterion  is  the  difference  in  stability.  Stannic 
acid  is  peptized  by  dilute  HC1,  HN03,  H2SO4  and  alkalies  while 
metastannic  acid  is  soluble  in  HC1,  insoluble  in  HNO3,  and  swells 
in  H2SO4.  As  a  matter  of  fact  one  can  have  a  whole  series  of 
stannic  acids.1  It  is  merely  a  question  of  how  carefully  one 
differentiates  properties.  By  rapid  dialysis  of  silicic  acid  solu- 
tions it  is  possible  to  get  much  higher  concentrations  than  by 
dialysis  because  the  silicic  acid  changes  less.2 

With  a  colloidal  solution  stabilized  by  an  adsorbed  ion,  the 
reversibility  of  the  coagulation  depends  upon  the  possibility  of 
washing  out  the  precipitating  agent  and  on  the  physical  state  of 
the  precipitate.  The  more  strongly  the  precipitate  is  adsorbed 
the  more  difficult  it  will  be  to  wash  it  out.  When  albumin  is 
precipitated  by  sodium  chloride,  the  coagulation  is  ordinarily 
reversible.  When  it  is  precipitated  by  the  salt  of  a  heavy  metal, 
the  coagulation  is  irreversible.  All  precipitates  tend  to  change 
on  standing,  becoming  coarser  or  more  firmly  agglomerated.  It 
may  then  happen  that  washing  out  the  precipitating  agent  will 
not  cause  peptization.  When  albumin  is  precipitated  by  calcium 
chloride;  the  coagulation  is  reversible  if  the  calcium  chloride  is 
washed  out  at  once;  but  is  not  reversible  if  the  precipitate  is 
allowed  to  stand.  If  a  precipitate  of  silver  chloride  be  allowed 
to  stand  over  night,  it  can  then  be  filtered  and  washed  much 
more  satisfactorily  than  when  first  precipitated. 

The  effect  of  temperature  changes  on  the  stability  of  a  colloidal 
solution  may  be  two-fold.  Raising  the  temperature  will  in- 
crease the  peptizing  action  of  the  solvent  thereby  increasing  the 
stability,  or  it  may  decrease  the  adsorption  of  the  stabilizing 
ion,  thus  making  the  solution  less  stable.  The  second  factor  is 
the  more  important  with  colloidal  solutions  stabilized  by  an 
adsorbed  ion.3  Thus  albumin  solutions  coagulate  on  heating 

.  *  VIGNON:  Jour.  Chem.  Soc.,  66,  833  (1889):  MECKLENBURG:  Zeit.  anorg. 
Chem.,  74,  207  (1912). 

2  ZSIGMONDY:  Van  Bemrnelen  Gedenkboek,  7  (1910). 

and  PICTON:  Jour.  Chem.  Soc.,  87,  1931  (1905). 


222  APPLIED  COLLOID  CHEMISTRY 

and  so  do  caseine  solutions.  When  a  solution  is  actually  frozen 
solid,  we  have  the  possibility  of  the  colloid  agglomerating.  If  it 
does  that,  the  colloidal  solution  will  not  be  formed  when  the 
mass  melts;  but  otherwise  it  may.  The  experiments  along  this 
.line  are  not  very  satisfactory  because  they  were  done  without  any 
knowledge  of  the  general  theory. l  The  main  result  is  about  what 
one  would  expect.  In  presence  of  a  very  small  amount  of  peptiz- 
ing  agent,  the  precipitation  is  likely  to  be  irreversible  because 
the  precipitate  agglomerates  on  standing.  If  there  is  a  fair  ex- 
cess of  peptizing  agent,  the  precipitation  will  be  reversible. 
This  is  of  course  especially  true  if  the  peptizing  agent  is  a  water- 
soluble  colloid.  Ehrenberg2  considers  that  the  pressure  due  to 
the  expansion  when  water  freezes  may  be  a  factor  in  causing 
coagulation. 

Where  the  adsorption  is  very  low,  it  may  require  a  very  high 
concentration  of  a  given  ion  to  cause  peptization.  This  occurs 
in  a  number  of  cases  of  peptization  by  alkalies  and  is  probably 
another  reason  why  people  have  considered  that  alkali  salts 
were  formed  and  not  colloidal  solutions. 

If  one  over-runs  the  point  of  electrical  neutrality,  the  suspen- 
sion may  become  stable  again  because  it  is  stabilized  by  preferen- 
tial adsorption  of  an  ion  having  the  opposite  sign.  If  one  adds 
a  little  less  than  the  equivalent  amount  of  a  dilute  potassium 
bromide  solution  to  a  dilute  silver  nitrate  solution,  one  gets  a 
positively-charged  colloidal  solution  of  silver  bromide  because  the 
silver  bromide  is  stabilized  by  adsorbed  silver  ions.  If  one  adds 
slightly  more  than  the  equivalent  amount  of  the  potassium  bro- 
mide solution,  there  is  obtained  a  negatively-charged  colloidal 
solution  of  silver  bromide,  stabilized  by  adsorbed  bromine  ions. 
If  more  bromide  solution  is  added,  the  silver  bromide  will  coagu- 
late because  there  will  come  a  point  at  which  the  slight  adsorption 
of  the  potassium  ion  counter-balances  the  adsorption  of  the 
bromine  ion.  In  other  words,  silver  bromide  precipitates  when 
there  is  a  distinct  excess  of  silver  nitrate;  stays  in  suspension  as  a 
positively  charged  colloid  when  there  is  only  a  slight  excess  of 

ILJUBAVIN:  Jour.  Chem.  Soc.,  68,  685  (1890);  LOTTERMOSER:  Zeit.  phys. 
Chem.,  60,  462  (1907);  Ber.  deutsch.  chem.  Ges.,  41,  3976  (1908);  Zeit. 
Elektrochemie,  17,  317  (1911);  FISCHER:  Zeit.  Kolloidchemie,  8,  292  (1911). 

2  Die  Bodenkolloide,  20  (1915). 


PROPERTIES  OF  COLLOIDAL  SOLUTIONS  223 

silver  nitrate,-  precipitates  when  the  suspension  becomes  electri- 
cally neutral/  stays  in  suspension  as  a  negatively  charged  colloid 
when  there  is  a  slight  excess  of  potassium  bromide/  and  precipi- 
tates when  there  is  a  distinct  excess  of  potassium  bromide.  At 
the  two  ends  of  this  series  there  is  also  the  possibility  of  true 
solution  of  silver  bromide  in  silver  nitrate  or  potassium  bromide. 
An  interesting  case  of  the  precipitation  of  a  colloid  by  an 
electrolyte  is  to  be  found  in  the  action  of  sea-water  on  muddy 
river  water.  Skey1  pointed  out  that  suspended  mud  is  precipi- 
tated by  electrolytes,  and  Waldie2  has  discussed  the  clearing  of 
the  water  of  the  Hooghly.  When  a  river  flows  into  the  ocean,  the 
current  becomes  less  and  some  of  the  suspended  mud  is  dropped 
on  this  account;  but  this  is  relatively  unimportant  in  many 
cases.  Schloesing3  called  attention  to  the  fact  that  the  forma- 
tion of  deltas  is  due  primarily  to  the  coagulation  of  the  suspension 
by  the  electrolytes  in  the  salt  water.  Another  interesting  case 
of  the  neutralization  of  an  electrical  charge  is  the  precipitation 
of  positively  charged  colloids  by  0  rays.4  Frischauer5  found  that 
j8  rays  increase  the  agglomeration  of  sulphur,  while  Doelter6 
reports  they  change  selenium  hydrosol  to  crystalline  selenium. 
Farmer  and  Parker7  report  that  ultra-violet  light  coagulates 
colloidal  platinum.  This  seems  to  be  in  line  with  the  action  of 
ultra-violet  light  in  causing  metals  to  emit  electrons.  It  would 
be  interesting  to  know  whether  the  action  is  confined  to  metals 
or  whether  ultra-violet  light  will  coagulate  any  negatively 
charged  sol.  Svedberg8  has  prepared  colloidal  solutions  ap- 
parently by  the  disintegrating  effect  of  ultra-violet  light;  but 
later  experiments9  indicate  that  the  silver  is  first  oxidized  and  then 

1  Chem.  News,  17,  160  (1868). 

2  Ibid.,  30,  37  (1874). 

3  Jour.  Chem.  Soc.,  24,  750  (1871). 

4  HARDY:  Jour.   Physiology,  29,  29   (1903);  JORISSEN  and    WOUDSTRA: 
Zeit.  Kolloidchemie,  8,  8  (1910);  HOBER:  Physikalische  Chemie  der  Zelle  und 
Gewebe,  332  (1911);  BURTON:  The  Physical  Properties  of  Colloidal  Solu- 
tions, 172  (1916). 

6  Comptes  rendus,  148,  1251  (1909). 

6  Das  Radium  und  die  Farben,  17,  95,  97  (1910). 

7  Jour.  Am.  Chem.  Soc.  35,  1524  (1913). 

8  Zeit.  Kolloidchemie,  6,  130  (1910);  SCHULZE:  Ibid.,  16,  136  (1914). 

9  NORDENSON:  Kolloidchemische  Beihefte,  7,  110  (1915). 


224  APPLIED  COLLOID  CHEMISTRY 

reduced.  Young  and  Pingree1  have  done  some  work  on  the 
action  of  light  on  colloidal  solutions.  There  is  a  very  important 
field  here  especially  with  reference  to  the  colors  of  gems;  but 
nobody  has  attacked  the  problem  systematically. 

2.  Sols  stabilized  by  Water. — When  we  come  to  colloidal 
solutions  peptized  by  liquids,  the  effect  of  salts  is  necessarily 
relatively  small.  Tannin  is  peptized  by  water  and  by  glacial 
acetic  acid.  Apparent  solubilities  for  tannin  vary  from  20-250 
parts  per  hundred  of  water;  but  Navassart2  has  shown  that  tan- 
nin is  miscible  in  all  proportions  with  water,  alcohol,  acetone  and 
glacial  acetic  acid.  The  apparent  solubility  has  been  due  to 
lack  of  wetting  of  the  tannin  powder  by  the  sol.  It  is  probable 
also  that  the  tendency  to  coagulate  increases  with  increasing 
concentration  and  the  presence  of  impurities  might  easily  cause 
agglomeration  and  precipitation  at  apparently  definite  concen- 
trations in  each  case.  Salts  have  relatively  little  effect  on  the 
stability  of  tannin  sols,  though  the  hydroxides  of  the  heavy  metals 
adsorb  tannin  strongly,  the  two  precipitating  together.  Gela- 
tine swells  in  cold  water  but  is  not  peptized  by  it.  It  is  peptized 
by  warm  water,  the  apparent  melting-point  or  maximum  gelation 
temperature  of  a  10  grams  per  100  cc  gelatine3  jelly  being  30.0°. 
Salts  in  relatively  high  concentration  have  an  effect,  and  the  anions 
may  be  arranged  in  the  following  order:  sulphate > citrate > 
tartrate  >  acetate  >  chloride  >  nitrate  >  bromide  >  iodide  >sul- 
phocyanate>benzoate>salicylate.  The  first  three  raise  the 
apparent  melting-point  while  the  others  lower  it.  From  these 
data  it  seems  probable  that  the  liquefaction  by  sodium  iodide, 
for  instance,  is  due  to  a  peptization  by  iodide  ion  superposed  on 
the  water  peptization.  Briggs4  has  shown  that  removing  iodide 
by  dialysis  causes  the  gelatine  to  set  again;  but  we  have  no  trust- 
worthy data  which  show  definite  adsorption  of  iodide  by  gelatine. 
Glycerine  and  sugar  behave  like  sulphates,  raising  the  apparent 
melting-point,  while  such  non-electrolytes  as  furfurol,  urea,  alco- 
hol, resorcinol,  hydroquinone,  and  pyrogallol  lower  the  apparent 
melting-point.  Here  again  we  have  empirical  observations 

1  Jour.  Phys.  Chem.,  17,  657  (1912). 

2  Kolloidchemische  Beihefte,  5,  299  (1913). 

3  SMITH:  Jour.  Am.  Chem.  Soc.,  41,  135  (1919). 

4  Jour.  Phys.  Chem.,  24,  74  (1920). 


PROPERTIES  OF  COLLOIDAL  SOLUTIONS  22,5 

and  no  real  knowledge.  Formaldehyde  hardens  gelatine  so 
that  it  is  not  peptized  by  water. 

By  soaps  we  usually  mean  sodium  palmitate,  stearate,  or 
oleate,  or  some  mixture  of  these  salts.  They  are  peptized  by 
water,  dilute  solutions  showing  a  rise  of  boiling-point  and  con- 
centrated solutions  practically  none  by  the  usual  method,  which 
is  in  error  owing  to  presence  of  air.  By  the  dew-point  method 
McBain1  finds  a  rise  of  0.2°  for  normal  potassium  stearate.  There 
is,  of  course,  a  certain  amount  of  hydrolysis  and  it  seems  probable 
that  a  sodium  oleate  solution  contains  sodium  oleate,  oleic  acid, 
and  caustic  soda,  the  first  two  of  which  are  not  in  solution.  The 
hydroxyl  ion  is  unquestionably  adsorbed  to  some  extent  by  the 
oleic  acid  or  sodium  oleate  or  by  both  and  it  is  possible  that  the 
caustic  soda  is  adsorbed  in  concentrated  solutions  It  should  be 
possible  to  verify  this  assumption  by  doing  ultra-filtration  of  soap 
and  analyzing  the  filtrate.  A  sufficient  addition  of  sodium 
salts  causes  the  precipitation  of  soap  and  this  is  made  use  of  in 
soap  manufacture.  We  do  not  know  to  what  extent  this  effect 
of  salt  is  due  to  a  direct  coagulation  of  the  soap  by  adsorbed 
ions  or  to  coagulation  owing  to  a  decrease  in  the  peptizing  action 
of  water.  Some  unpublished  experiments  by  Clowes  seem  to 
indicate  that  the  concentration  of  sodium  ions  is  an  important 
factor,  though  Kurzmann2  finds  that  equimolecular  solutions  of 
potassium  carbonate  and  hydroxide  behave  alike  and  not  equiva- 
lent solutions. 

3.  Sols  stabilized  by  Non-electrolytes  or  by  a  Second  Colloid. 
A  colloid  which  is  stabilized  by  a  strongly-adsorbed  non-elec- 
trolyte such  as  sugar  will  behave  very  much  like  a  water-soluble 
colloid.  There  has  been,  relatively  little  work  done  on  such  sys- 
tems. There  are  many  interesting  possibilities,  however,  when 
we  consider  the  mutual  action  of  two  colloidal  solutions. 

If  we  mix  colloidal  solutions  of  two  substances  each  of  which 
adsorbs  the  other,  the  mutual  adsorption  will  have  some  effect 
on  the  adsorption  of  the  peptizing  agent.  If  it  causes  a  decrease 
in  the  adsorption  of  the  peptizing  agent,  this  means  a  decreasing 
stability  of  the  colloidal  solution  and  may  lead  to  precipitation 
over  some  range  of  relative  concentrations.  In  case  this  occurs, 

1  Jour.  Am.  Chem.  Soc.,  42,  432  (1920). 

2  Kolloidchemische  Beihefte,  5,  427  (1914). 

15 


226  APPLIED  COLLOID  CHEMISTRY 

we  shall  have  colloidal  solutions  of  decreased  stability  when 
either  colloid  predominates  sufficiently,  and  precipitation  for  inter- 
mediate relative  concentrations.  The  range  over  which  precipi- 
tation occurs  may  be  great  or  small  depending  on  the  ease  with 
which  the  peptizing  agent  is  displaced  or  neutralized.  The  most 
familiar  case  of  such  precipitation  is  when  we  are  dealing  with  two 
colloids  having  opposite  electrical  charges.  This  point  was 
studied  carefully  by  Biltz1  and  the  general  statement  is  to  be 
found  in  all  the  text-books  on  colloid  chemistry.  The  negatively 
charged  colloids  studied  by  Biltz  were  platinum,  gold,  selenium, 
cadmium  sulphide,  antimony  sulphide,  arsenic  sulphide,  stannic 
oxide,  molybdenum  blue  (MO3O8),  tungsten  blue  ("W^Oa),  and 
vanadium  pentoxide.  The  positive  colloids  were  the  oxides  of 
iron,  aluminum,  chromium,  thorium,  zirconium,  and  cerium. 
Precipitation  occurs  when  each  positive  colloid  is  mixed  with 
each  negative  colloid  in  proper  proportions.  Since  complete 
neutralization  takes  place  only  when  one  sol  has  adsorbed  the 
amount  of  the  sol  carrying  an  equivalent  amount  of  the  ion  having 
the  opposite  charge,  it  follows  that  the  amount  of  one  sol  neces- 
sary to  precipitate  a  given  amount  of  another  sol  will  vary  with 
the  degree  of  adsorption.  It  will  therefore  be  a  specific  property 
and  not  not  an  additive  one.  This  can  be  tested  experimentally 
on  data  by  Biltz  given  in  Table  XLV.2 

Alumina  is  more  effective  than  chromic  oxide  in  precipitating 
antimony  sulphide  and  much  less  effective  in  precipitating  arsenic 
sulphide.  The  alumina  must  therefore  be  adsorbed  more  by 
antimony  sulphide  than  chromic  oxide,  while  the  reverse  must 
be  true  for  arsenic  sulphide.  Cerium  oxide  is  less  effective  than 
either  of  these  in  precipitating  the  sulphides  of  antimony  and 
arsenic.  The  phenomenon  is  thus  specific,  varying  with  the 
nature  of  the  two  colloids.  This  seems  not  to  have  been  realized 
before.  In  fact  Freundlich  says  definitely3  that  one  seems  to 
find  approximately  the  same  order  regardless  of  what  sol  is  to  be 
precipitated.  This  statement  is  true,  but  it  missed  the  impor- 
tant thing  in  the  experiments,  which  was  that  the  order  was  not 
always  the  same. 

1  Ber.  deutsch.  chem.  Ges.,  37,  1095  (1904).  * 

2  FREUNDLICH:  Kapillarchemie,  445  (1909). 

3  FREUNDLICH:  Kapillarchemie,  445  (1909). 


PROPERTIES  OF  COLLOIDAL  SOLUTIONS .  227 

So-called  irregular  series1  have  been  observed  when  a  suspen- 
sion of  a  positively-charged  colloid  is  added  to  a  suspension  of  a 
negatively-charged  colloid.  When  increasing  concentrations  of 
aluminum  sulphate  were  added  to  a  negatively-charged  solution 
of  mastic,  there  was  at  first  no  precipitation  and  the  suspension 
was  charged  negatively  because  the  mastic  was  present  in  excess. 
At  higher  concentrations  of  aluminum  sulphate,  the  suspension 
became  electrically  neutral  and  complete  precipitation  took  place. 
At  still  higher  concentrations  the  mastic  was  held  in  suspension  by 
the  hydrolyzed  alumina  and  the  suspension  was  charged  posi- 

TABLE  XLV. — PRECIPITATION  OF  NEGATIVE  COLLOIDS 

1.4  mg  gold  completely  precipitated  by 

CeOz  Fe2O3  ThO2  ZrO2  Cr2O3  A12OS 

4  3  2.5  1.6  0.3         0.1-0.2mg 

28  mg  Sb2S3  completely  precipitated  by 

Fe2Os       ThO2        CeO2        ZrO2       Cr2O3       A12O3 

32  20  11  6.5  3.0  2.0mg 

24  mg  As2s3  completely  precipitated  by 
Fe2O3  ThO2  CeO2  ZrO2  A12O3  Cr2O8 

13  6  4  2  2  0.5  rag 

tively.  At  still  higher  concentrations  of  aluminum  sulphate, 
complete  precipitation  occurred.  This  apparently  abnormal 
result  is  due  to  the  fact  that  the  experiment  is  not  exactly  what  it 
purports  to  be,  the  addition  of  one  colloidal  solution  to  another. 
Instead  of  adding  a  colloidal  solution  of  alumina,  there  was  added 
a  so-called  solution  of  aluminum  sulphate,  which1  hydrolyzed 
to  colloidal  alumina  and  sulphuric  acid,  with  possibly  some 
aluminum  sulphate  left  unchanged  especially  in  the  higher  con- 
centrations. The  amount  of  free  sulphuric  acid  is  always  equiva- 
lent to  the  amount  of  alumina  and  the  two  concentrations 
increase  proportionally.  The  so-called  abnormal  precipitation  at 
the  higher  concentrations  is  merely  a  precipitation  by  sulphate 
ions  and  is  absolutely  normal. 

Since  a  colloid  peptized  by  water  may  be  charged  positively  or 
negatively,  there  is  no  reason  why  it  should  riot  precipitate  another 
colloid  under  suitable  conditions.  We  usually  consider  the  col- 

1  NEissERand  FRIEDEMANN:  Zeit.  angew.  Chem.,  1906,  1953;  BECHHOLD: 
Zeit.  phys.  Chem.,  48,  285  (1904);  BUXTON  and  TEAGUE:  Ibid.,  67,  47,  64 
(1907);  FREUNDLICH:  Kapillarchemie,  402  (1909). 


228  .  APPLIED  COLLOID  CHEMISTRY 

loids  peptized  by  water  solely  as  protecting  colloids  but  this  is 
clearly  an  inadequate  view  as  is  shown  by  the  experimental  data. 
Years  ago  Schulze1  pointed  out  that  small  amounts  of  a  gelatine 
solution  were  as  effective  as  lime  or  alum  in  causing  the  rapid 
sedimentation  of  clay  and  that  addition  of  minute  quantities  of 
gelatine  to  barium  sulphate  simplified  the  question  of  filtration 
and  washing  very  much.  He  of  course  gave  no  adequate  explana- 
tion of  the  phenomenon  and  the  real  explanation  was  given  by 
Billitzer2  who  called  attention  to  the  fact  that  while  gelatine, 
agar-agar,  etc.,  ordinarily  check  the  precipitation  of  colloidal 
solutions  by  electrolytes,  small  amounts  of  these  same  substances 
may  have  a  precipitating  action.  This  can  be  detected  even 
when  the  gelatine  produces  no  precipitation  itself.  Billitzer 
cites  the  experiments  of  Neisser  and  Friedemann  in  which  it  was 
found  that  a  mastic  emulsion  containing  a  trace  of  gelatine  was 
precipitated  more  readily  by  sodium  chloride  than  when  no 
gelatine  is  present.  Billitzer  finds  that  gelatine  precipitates 
such  negative  colloids  as  antimony  sulphide  and  arsenic  sulphide 
in  acid  or  neutral  solution,  but  does  not  precipitate  positively 
charged  sols  such  as  hydrous  ferric  oxide.  Gelatine  in  ammoni- 
avcal  solution  precipitates  hydrous  ferric  oxide,  though  no  precipi- 
tation occurs  if  ammonia  is  added  to  a  mixture  of  gelatine  and 
ferric  oxide.  Bismarck  brown,  which  is  a  positive  colloid,  is 
precipitated  by  an  alkaline  gelatine  solution,  while  eosine  is 
precipitated  by  an  acidified  gelatine  solution. 

Graham3  points  out  that  colloidal  silica  is  precipitated  by 
gelatine,  the  composition  of  the  precipitate  varying  between  56 
and  92  parts  of  gelatine  per  hundred  of  silicic  acid. 

Lumiere  and  Seyewetz4  have  studied  the  action  of  salts  of 
chromium  upon  gelatine  because  of  its  importance  for  the  theory 
of  chrome  tanning.  If  a  suitable  amount  of  chrome  alum  is 
added  to  a  gelatine  sol  or  if  sheets  of  gelatine  are  soaked  long 
enough  in  a  chrome  alum  solution,  the  gelatine  becomes  insoluble 
even  in  boiling  water  and  does  not  putrefy  on  standing  as  the 
original  gelatine  would  have  done.  They  found  first  that  gela- 

1  Pogg.  Ann.,  129,  369  (1866). 

2  Zeit.  phys.  Chem.,  51,  145  (1905). 
8  Jour.  Chem.  Soc.,  16,  246  (1862). 

4  Bull.  Soc.  chim.  Paris,  (3)  29,  1077  (1903). 


PROPERTIES  OF  COLLOIDAL  SOLUTIONS  229 

tine  could  be  rendered  insoluble  by  treatment  with  almost  any 
salt  of  the  formula  Cr2X6:  sulphate,  sulphite,  nitrate,  chloride, 
fluoride,  acetate,  formate,  citrate,  or  lactate.  Since  it  had  been 
shown  by  Namias  that  the  power  of  chrome  alum  to  make  gela- 
tine insoluble  was  increased  by  addition  of  alkali  up  to  the  point 
where  precipitation  of  hydrous  chromic  oxide  occurred,  experi- 
ments were  made  with  the  green  basic  chromic  sulphate  of  Recoura. 
Gelatine  treated  with  a  solution  of  this  salt  resisted  repeated 
washing  with  boiling  water  better  than  gelatine  which  had  been 
treated  with  a  solution  which  was  not  basic.  On  the  other  hand, 
gelatine  is  not  made  insoluble  by  treatment  with  a  so-called 
solution  of  chromic  oxide  in  potassium  hydroxide;  but  this  is 
probably  due  to  the  excess  of  alkali.  If  the  proper  amount  of 
ammonia  is  added  to  gelatine  and  then  a  chrome  alum  solution, 
the  gelatine  becomes  insoluble  in  boiling  water;  but  this  is  not 
the  case  if  ammonia  is  present  in  excess.  The  gelatine  is  insolu- 
ble when  it  contains  2.5-25  grams  of  chrome  alum  per  hundred 
of  gelatine. 

Experiments  with  chromic  sulphate,  chloride,  nitrate,  and 
acetate  showed  that  approximately  the  same  quantity  of  chromic 
oxide  was  necessary  to  make  the  gelatine  insoluble  irrespective 
of  the  nature  of  the  acid.  Since  this  implies  that  only  the  chromic 
oxide  counts,  experiments  were  made  to  see  what  became  of  the 
acid  radical.  Sheets  of  gelatine  were  immersed  in  a  known 
chromic  sulphate  solution  and  the  solution  analyzed  afterwards. 
The  ratio  of  chromium  to  sulphate  in  the  solution  was  practi- 
cally the  same  before  and  after  treatment  with  gelatine,  which 
meant  apparently  that  the  chromic  sulphate  was  taken  up  as  a 
whole.  It  could  not  be  the  chromic  sulphate  as  such  which 
made  the  gelatine  insoluble  because  the  same  result  could  be 
obtained  when  chrome  alum  was  added  to  an  ammoniacal  gela- 
tine. When  the  gelatine  containing  chromic  sulphate  is  washed 
repeatedly  with  boiling  water,  acid  is  extracted  very  slowly;  but 
after  some  washings  the  gelatine  swells  and  a  little  is  carried  off 
in  the  wash  water,  leaving  the  residue  relatively  richer  in  chro- 
mium, the  chromic  oxide  content  running  up  to  5.4-4.6  grams  per 
hundred  grams  of  gelatine.  If  the  gelatine  is  treated  with  dilute 
alkali,  it  is  possible  to  remove  the  acid  without  causing  any  swell- 
ing or  any  solution  of  the  gelatine.  When  the  gelatine  is  treated 


230  APPLIED  COLLOID  CHEMISTRY 

in  this  way  the  maximum  amount  of  chromium  found  in  the 
gelatine  is  3.3-3.5  grams  Cr2O3  per  hundred  grams  of  gelatine. 

The  explanation  seems  to  be  that  the  chromic  oxide  makes  the 
gelatine  insoluble  and  that  the  acid  is  adsorbed  strongly  by  the 
chromic  oxide,  the  gelatine,  or  both.  When  the  acid  is  washed 
out  at  high  temperatures,  it  causes  a  swelling  of  the  gelatine. 
In  fact,  an  insoluble  gelatine,  which  has  been  freed  from  acid,  swells 
rapidly  and  finally  dissolves  if  boiled  with  water  containing 
small  amounts  of  acid  or  alkali.  If  the  acid  is  adsorbed  by  the 
chromic  oxide  in  the  gelatine,  it  would  be  interesting  to  know 
whether  there  is  a  time  factor  and  whether  after  standing  for  a 
month  it  would  be  easier  to  wash  out  the  acid. 

Coming  back  to  the  general  problem,  if  the  ratio  of  gelatine  to 
the  other  colloid  is  increased  sufficiently,  we  shall  pass  through 
the  precipitation  range  into  the  range  where  the  colloidal  solu- 
tion is  stabilized  by  gelatine  and  then  behaves  more  like  a  water- 
soluble  colloid.  Colloidal  gold  and  colloidal  silver  solutions, 
when  stabilized  by  gelatine  can  be  evaporated  and  redissolved 
because  the  gelatine  prevents  the  irreversible  agglomeration. 
When  less  gelatine  is  used,  it  may  retard,  though  not  prevent,  the 
change  of  red  colloidal  gold  to  blue.  Zsigmondy^defines  as  the 
gold  number  the  number  of  milligrams  of  a  protecting^coiloid 
which  just  prevents  the  color  change  in  a  10  cc  red  gold  solution, 
containing  0.0053-0.0058  percent  gold,  when  one  cubic  centi- 
meter of  a  ten  percent  sodium  chloride  solution  is  added. 
course  a  strongly  adsorbed  non-electrolyte,  such  as  sugar,  will 
act  similarly  to  gelatine;  but,  in  most  cases,  the  adsorption  is  so 
much  less  that  these  substances  are  only  interesting  theoretically 
as  stabilizers.  Some  of  his  results  are  given  in  Table  XL VI. 

Most  of  the  stable  colloidal  solutions  depend  for  their  stability 
on  the  presence  of  a  so-called  protecting  colloid.  There  are 
several  other  ways  in  which  this  property  can  be  made  useful. 
If  we  have  colloids  like  the  so-called  zinc  and  magnesium  resinates 
which  are  peptized  by  benzene,  we  can  use  these  as  protecting 
colloids  to  stabilize  pigments  and  thus  form  colloidal  solutions 
which  could  not  otherwise  exist  in  benzene.  There  are  other 
cases  where  one  does  not  insist  on  a  colloidal  solution  but  wishes  a 
fine-grained  precipitate.  The  most  important  case  of  this  sort 

1  Zeit,  anal  Chem.,  40,  697  (1901). 


PROPERTIES  OF  COLLOIDAL  SOLUTIONS  231 

technically  is  the  photographic  emulsion,  which  consists  chiefly 
of  silver  bromide  precipitated  in  presence  of  gelatine,  so  as  to  get 
a  fine-grained  precipitate,  which  is  afterwards  given  the  desired 
properties  by  agglomerating  through  suitable  treatment.  The 
gelatine  plays  the  additional  part  of  a  depolarizer. 

TABLE  XL VI. — GOLD  NUMBER  FOR  DIFFERENT   COLLOIDS 


Colloid 

Gold  number 

Gelatine                                         

0   005-0  01 

Caseiiie 

0   01 

Eg0"  albumin                     .  .    

0.06-0.3 

Gum  arabic                                               •    •        •  •  •      

0  15-0.25 

Gum  tragacanth 

2 

Dextrin                                          .  .      

10-20 

Potato  starch 

25 

Sodium  oleate                               

0.4-1.0 

Sodium  stearate  (added  boiling  hot) 

0  001 

Sodium  stearate  (added  at  60°)  

10 

Even  though  the  gelatine  or  similar  substance  may  not  be 
present  in  sufficient  amount  to  prevent  precipitation,  its  pres- 
ence may  make  itself  known  by  cutting  down  the  agglomeration 
of  the  colloidal  precipitate.  When  caseine  is t  precipitated  from 
cow's  milk  by  acid,  it  comes  down  in  relatively  coarse  lumps 
which  is  one  reason  why  cow's  milk  is  bad  for  infants.  By  adding 
gelatine1  to  the  milk,  the  growth  of  the  curdy  precipitate  can  be 
prevented.  In  human  milk  a  protecting  colloid  is  present  and 
consequently  coarse  curds  are  not  formed.  This  property  of 
gelatine  and  other  strongly  adsorbed  substances  of  keeping  down 
the  size  of  precipitates  is  made  use  of  in  the  preparation  of  many 
colloidal  solutions.  By  varying  the  amount  of  gelatme  one  can 
vary  the  fineness  of  the  precipitate.  The  addition  of  gelatine  to 
ice  cream  prevents  the  formation  of  coarse  crystals.  In  electro- 
lytic refining  of  metals  and  in  electroplating,  addition  agents  are 
used  to  make  the  deposited  metal  less  coarsely  crystalline.  An 
addition  agent  must  not  be  carried  away  from  the  cathode  by  the 
current  and  must  be  adsorbed  by  the  precipitating  metal. 

1  ALEXANDER:  Jour.  Soc.  Chem.  Ind.,  28,  280  (1909). 


232  APPLIED  COLLOID  CHEMISTRY 

Gelatine  is  used  in  lead  refining  and  has  a  great  effect  in  an  acidi- 
fied copper  sulphate  solution.  Many  of  the  essential  oils  have 
proved  satisfactory.  Glucose  can  also  be  used  in  some  cases, 
not  because  it  is  a  colloid,  but  because  it  is  adsorbed  strongly  by 
the  precipitating  metal. 

In  the  case  of  two  colloids,  each  peptized  by  water,  the  mutual 
adsorption  may  cut  down  the  adsorption  of  water  sufficiently  to 
cause  precipitation.  This  seems  to  be  the  most  plausible  ex- 
planation for  the  behavior  of  gelatine  and  tannin.  The  only 
difficulty  is  that  the  adsorption  is  more  marked  when  the  gelatine 
is  positive  and  the  tannin  negative,  which  is  more  what  one  would 
expect  if  the  two  were  colloids  peptized  by  ions.  There  seem 
to  be  at  least  two  possible  ways  out  of  this  difficulty.  It  may  be 
that  gelatine  is  not  really  peptized  by  water1  and  that  we  are 
dealing  with  a  case  of  ion  peptization  where  the  range  of  in- 
stability is  very  small.  The  other  possibility  is  that  we  get  a 
different  adsorption  when  the  colloids  are  charged  oppositely 
and  that  the  resulting  complex  is  less  readily  peptized  by  water 
under  those  conditions.  This  is  not  an  impossibility,  as  is 
shown  by  the  fact,  previously  mentioned,  that  ammoniacal  gela- 
tine precipitates  colloidal  ferric  oxide,  whereas  there  is  no  pre- 
cipitation if  the  gelatine  and  ferric  oxide  sols  are  mixed  before 
the  ammonia  is  added.  There  seems  to  be  no  way  at  present  to 
decide  this  question. 

Tannin  and  basic  dyes  give  a  precipitate  but  the  range  over 
which  this  occurs  is  very  limited  and  in  order  to  make  use  of  tliis 
property  in  dyeing,  it  is  customary  to  fix  the  tannin  with  a  salt 
of  antimony.  Of  course,  in  so  far  as  the  water  peptization  is 
cut  down,  the  mixture  will  tend  more  and  more  to  behave  like  a 
colloid  peptized  by  ions. 

Tiebackx^  finds  that  when  a  half  percent  gelatine  solution  is 
mixed  with  a  four  percent  gum  arabic  solution,  the  mixture  is 
precipitated  readily  by  acids.  If  the  acid  is  washed  out,  the 
coagulum  can  readily  be  dispersed  in  water.  Tiebackx  points 
out  that  the  properties  of  a  gelatine  and  gum  arabic  mixture  are 
almost  those  of  a  globulin  or  of  caseine.  "Caseine  is  peptized 
by  dilute  alkalies  and  somewhat  stronger  acids,  the  precipitation 

1  Cf.  LOEB:  Jour.  Gen.  Physiology,  1,  365  (1919). 

2  Zeit.  Kolloidchemie,  8,  198,  238  (1911), 


PROPERTIES  OF  COLLOIDAL  SOLUTIONS  233 

range  lying  between  these  two  limits.  Sulphuric  acid  is  also 
adsorbed  by  caseine  somewhat  more  strongly  than  hydrochloric 
acid.  With  globulin  and  caseine  the  acids  are  also  displaced  by 
salts  as  can  be  shown  by  means  of  methyl  orange.  The  dis- 
placement of  acid  also  involves  a  peptization,  for  van  Dam1  has 
shown  that  presence  of  five  percent  sodium  chloride  increases 
the  amount  of  caseine  peptized  by  a  given  concentration  of  acid." 

Apparently  gelatine  and  agar-agar  are  two  practically  non- 
miscible  liquids,  for  Beijerinck2  reports  that  it  is  impossible  to 
mix  a  hot  ten  percent  gelatine  solution  with  a  hot  two  percent 
agar  solution,  one  of  the  liquids  being  emulsified  in  the  other  when 
the  two  are  shaken  together.  The  phenomenon  is  even  more 
marked  when  ten  percent  soluble  starch  is  shaken  with  ten 
percent  gelatine. 

Biltz3  obtained  no  precipitation  upon  mixing  any  two  of  his 
positively  charged  sols  and  no  precipitation  on  mixing  any  two 
of  the  negatively-charged  sols  except  when  gold  or  selenium  was 
mixed  with  the  sulphides  and  then  chemical  action  probably 
took  place.  Biltz  concluded  that  sols  of  like  charge  had  no 
effect  upon  each  other;  but  this  does  not  follow,  because  adsorp- 
tion of  one  colloid  by  the  other  would  not  neutralize  the  electrical 
charge  and  therefore  would  not  cause  precipitation.  We  know, 
however,  that  cases  of  adsorption  are  not  limited  to  colloids  or 
electrolytes  having  opposite  signs.  Charcoal  adsorbs  both 
bases  and  acids.  Silver  bromide  adsorbs  silver  ions  or  bromine 
ions  as  the  case  may  be.  There  is  therefore  no  theoretical  reason 
why  precipitated  hydrous  ferric  oxide  might  not  adsorb  chromic 
oxide  and  vice-versa.  If  the  precipitated  substance  will  do  this 
there  is  no  reason  why  the  peptized  substance  should  not. 
Nagel4  has  shown  recently  that  this  does  occur  and  that  it 
accounts  for  the  behavior  of  mixtures  of  chromic  and  ferric  salts 
with  excess  of  alkali.  Hydrous  chromic  oxide  is  peptized  by 
caustic  potash  while  hydrous  ferric  oxide  is  not.  If  the  chromium 
salt  is  present  in  large  amount  relatively  to  the  iron  salt,  the 
ferric  oxide  will  adsorb  the  peptized  chromic  oxide  and  be  pep- 

1  Van  Bemmelen  Gedenkboek,  104  (1910). 

2  Zeit.  Kolloidchemie,  7,  16  (1910). 

3  Ber.  deutsch.  chem.  Ges.,  37,  1095  (1904). 
*  Jour.  Phys.  Chem.,  19,  331  (1915). 


234  APPLIED  COLLOID  CHEMISTRY 

tized  by  it,  going  apparently  into  solution.  If  the  ferric  salt  is 
present  in  excess,  it  will  adsorb  the  peptized  chromic  oxide, 
carrying  it  out  of  the  liquid  phase.  It  is  to  be  noticed  that  the 
chromic  oxide,  when  in  excess,  acts  as  a  so-called  protecting  col- 
loid to  the  iron  oxide.  Other  cases  of  the  same  type  could  be 
cited.  The  fact  that  gelatine  protects  ferric  oxide  when  added 
before  the  ammonia  shows  that  it  must  have  been  adsorbed  by 
the  ferric  oxide.  Bechhold1  has  shown  that  an  ultra-filter 
which  will  stop  Prussian  blue  and  will  let  arsenic  sulphide  pass, 
holds  back  the  arsenic  sulphide  when  mixed  with  Prussian  blue. 
Although  both  are  negatively  charged  sols,  it  seems  evident  that 
there  is  a  mutual  adsorption.  On  the  other  hand,  Bechhold 
found  that  hemoglobin  can  be  separated  from  Prussian  blue  by 
filtration  through  a  suitable  ultra-filter,  the  hemoglobin  going 
through  the  filter.  In  this  case  there  is  no  appreciable  adsorption. 

Since  the  precipitation  of  one  colloid  by  another  may  take 
place  within  a  relatively  narrow  range  of  concentrations,  it  is 
not  surprising  that  people  have  postulated  the  existence  of  defi- 
nite chemical  compounds  in  order  to  account  for  the  precipitation. 
The  literature  on  this  subject  is  in  a  worse  state  even  than  that  on 
the  so-called  basic  salts.  To  cite  a  few  cases  out  of  many,  ferric 
arsenate,  aluminum  silicate,  aluminum  stannate,  stannic  phos- 
phate, and  cupric  eosinate  are  not  formed  under  ordinary  condi- 
tions by  precipitation  from  solution.  The  precipitation  is  due  to 
the  mutual  coagulation  of  two  colloids.  If  each  adsorbs  the 
other  pretty  strongly,  it  may  easily  happen  that  the  precipitate 
can  be  peptized  without  change  of  composition  under  certain 
conditions,  a  behavior  which  one  usually  associates  with  com- 
pounds. The  color  lakes  are  an  especially  fruitful  field  for  imagin- 
ary formulas.  Unpublished  work  by  Mr.  Basseches  of  the 
Bureau  of  Standards  shows  that  no  ferric  tannate  is  formed  under 
ordinary  conditions. 

The  purple  of  Cassius  is  a  typical  case.  It  is  usually  prepared 
by  the  reaction  of  gold  chloride  with  stannous  chloride  and  con- 
sists of  colloidal  gold  adsorbed  by  stannic  oxide.  Since  it  could 
apparently  be  carried  into  solution  by  ammonia  and  since  mercury 
did  not  extract  gold  from  it,  Berzelius  was  positive  that  it  must 
be  a  compound  and  he  wrote  for  it  the  formula,  Au2O2.Sn2O3. 

1  Zeit.  phys.  Chem.  60,  299  (1907). 


PROPERTIES  OF  COLLOIDAL  SOLUTIONS  235 

xH2O.  The  final  proof  that  it  was  a  colloidal  precipitate  was 
given  by  Zsigmondy1  who  made  it  directly  from  colloidal  gold 
and  colloidal  stannic  oxide.  Owing  to  the  intensity  of  the  color, 
the  formation  of  the  purple  of  Cassius  may  be  used  as  a  test  for 
gold,2  sensitive  to  one  part  in  one  hundred  million.  It  has  also 
been  possible  to  obtain  similar  precipitates  with  alumina,  barium 
sulphate,  mercurous  chloride,  etc.3  A  corresponding  brown4 
and  red5  precipitate  has  been  obtained  with  platinum.  When 
purple  of  Cassius  is  made  with  zirconia,  the  precipitate  is  not 
peptized  by  ammonia  and  aqua  regia  does  not  extract  the  gold.6 

*  Liebig's  Ann.,  301,  375  (1898). 

2  ROSE:  Jour.  Chem.  Soc.,  64  II,  245  (1893). 

3  SCHWARZ:  Jour.    Chem.,   26,   205    (1873);    MULLER:    Ibid.,   48,   352 
(1885);  KRUSS:  Ibid,.  62,  16  U887);  ANTONY  and  LUCCHESI:  Ibid.,  72  II,  43 
(1897);  MARSHALL  and  WELKER:  Jour.  Am.  Chem.  Soc.,  35,  822,  823  (1913). 

4  DELACHANAL  and  MERMET:  Comptes  rendus,  81,  370  (1875). 

5  WOHLER:  Zeit.  Elektrochemie,  14,  634  (1908). 
6BiLTz:  Ber.  deutsch.  chem.  Ges.,  37,  1095  (1904). 


CHAPTER  VIII 

GELATINOUS  PRECIPITATES  AND  JELLIES 

i 
STRUCTURE 

When  we  add  caustic  soda  to  a  ferric  chloride  or  an  aluminum 
chloride  solution,  we  get  a  precipitate  which  varies  from  gelati- 
nous to  sandy,  depending  on  the  temperature,  being  more  sandy 
the  higher  the  temperature  of  precipitation.  In  this  case  we  have 
the  gelatinous  precipitate  and  a  supernatant  liquid.  If  we  add 
caustic  potash  in  excess  to  a  chromic  sulphate  solution,  we  may 
get  a  gelatinous  precipitate  with  a  supernatant  liquid  or  we  may 
get  the  whole  mass  setting  to  a  jelly,  which  may  afterwards 
contract  and  squeeze  out  liquid.1  To  avoid  confusion  in  the 
future  it  will  be  simpler  to  use  the'  word  jelly  only  for  those  cases 
where  there  is  no  supernatant  liquid  at  first.  This  is  in  accord 
with  general  usage  for  one  speaks  of  a  gelatine  solution,  for  in- 
stance, setting  to  a  jelly;  but  some  writers  have  been  so  careless  in 
their  use  of  the  word  that  it  has  not  always  been  clear  just  what 
they  did  mean.  Gel  is  the  inclusive  term,  gelatinous  precipitates 
and  jellies  being  two  forms  of  gels. 

A  gelatinous  precipitate  is  always  viscous  and  contains  liquid; 
it  may  be  plastic.  No  one  has  any  difficulty  in  recognizing  a 
gelatinous  precipitate;  but  we  are  not  at  all  clear  as  to  what  gives 
it  these  physical  properties.  In  the  case  of  gelatinous  ferric  oxide 
there  is  every  reason  to  suppose  that  none  of  the  water  is  com- 
bined to  form  a  definite  chemical  compound.  Vapor  pressure 
experiments  on  silicic  acid  jellies  show  no  evidence  of  any  definite 
chemical  compound  between  silica  and  water.  Le  Chatelier2 
believes  that  a  silicic  acid  jelly  is  merely  grains  of  anhydrous 
silica  and  water.  He  claims  to  have  proved  this  by  polishing 
metals  with  colloidal  silicic  acid;  but  this  is  not  conclusive  be- 
cause anhydrous  silica  might  have  been  formed  as  a  result  of  the 

1  Syneresis  is  the  technical  term. 

2  La  Silice  et  les  Silicates,  76  (1914). 

236 


GELATINOUS  PRECIPITATES  AND  JELLIES  237 

pressure  used  in  polishing.  A  better  line  of  attack  is  to  consider 
whether  grains  of  sand  mixed  with  water  could  give  a  precipitate 
having  the  properties  of  gelatinous  silicic  acid.  Presence  of 
water  will  make  sand  plastic,  and  addition  of  an  excess  of  water 
will  give  a  mass  of  wet  sand  with  supernatant  liquid,  which  is 
good  so  far  as  it  goes.  If  the  sand  is  shaken  with  an  excess  of 
water,  the  grains  will  be  separated  and  will  afterwards  settle  at  a 
given  rate  depending  on  the  conditions.  That  does  not  happen 
with  the  silicic  acid  jelly.  There  is  a  marked  coalescence  which 
we  do  not  get  with  sand  and  water  so  far  as  we  know.  We  must 
therefore  assume  one  of  two  things.  Either  the  sand  grains  are 
held  together  extraordinarily  firmly  by  water  when  they  are 
very  fine  or  there  is  some  other  factor  comes  in.  The  first  ex- 
planation cannot  be  the  right  one  because,  if  it  were,  one  ought 
then  to  be  able  to  get  a  gelatinous  precipitate  of  any  colloid  at 
ordinary  temperatures  without  much  difficulty,  which  is  not  the 
case.  We  never  get  gelatinous  gold  and  while  we  can  get  gelat- 
inous calcium  carbonate,  we  have  to  do  it  in  a  very  special  way. 
Consequently,  Le  Chatelier's  hypothesis  cannot  be  accepted 
without  modification.  We  are  not  dealing  with  a  solid  solution 
because  then  the  vapor  pressure  would  be  independent  of  the 
previous  history  of  the  sample. 

It  seems  to  me  that  Oden's  experiments  on  sulphur  are  very 
important  in  their  bearing  on  this  problem.  Oden1  found  that 
the  physical  properties  of  sulphur  precipitated  from  colloidal 
solution  varied  very  markedly  with  the  electrolyte  used  for 
precipitation.  It  came  down  as  a  hard  precipitate  with  potas- 
sium salts,  fine-grained  with  copper  sulphate,  plastic  with  barium 
salts,  fluid  with  hydrochloric  acid,  and  slimy  with  other  salts. 
Similar  but  less  marked  phenomena  have  been  observed  with 
colloidal  ferric  oxide,2  which  comes  down  more  sandy  when 
precipitated  by  hydrochloric  acid  than  when  precipitated  by 
sulphuric  acid.3  In  the  case  of  sulphur  we  are  not  very  far 
below  the  melting-point  and  we  know  that  it  is  a  simple  matter  to 
supercool  molten  sulphur  so  as  to  get  a  plastic  mass  at  ordinary 

1  Der  kolloide  Schwefel,  134,  157  (1912). 

2  GIOLITTI:  Gazz.  chim.  ital,  35  II,  181  (1905);  36  II,  157,  433  (1906). 

3  For  alumina  with  chlorides  and  sulphates  see  ROSE  :  Zeit.  Kolloidchemie, 
16,  1  (1914). 


238  APPLIED  COLLOID  CHEMISTRY 

temperatures.  The  simplest  assumption  is  that  the  sulphur  pre- 
cipitated from  colloidal  solution  may  contain  enough  water  and 
electrolytes  to  keep  it  as  a  supercooled  viscous  liquid  the  drops 
of  which  have  coalesced  presumably  to  form  filaments  or  perhaps 
nets,  thus  giving  it  a  structure.  The  only  apparent  difference 
between  the  sulphur  and  the  ferric  oxide  is  that  we  never  get  as 
fluid  a  mass  with  the  iron  oxide  as  with  the  sulphur;  but  that  may 
be  due  to  the  difference  in  the  melting-points.  It  seems  possible 
therefore  that  a  gelatinous  precipitate  consists  of  partially 
coalescing  drops  or  films  of  a  viscous  liquid  containing  more  or 
less  of  the  mobile  liquid  adsorbed  or  held  in  other  ways.  The 
super-cooled  liquid  is  kept  from  solidifying  by  the  water  or  the 
electrolyte  or  both.  While  this  does  not  seem  a  very  satisfactory 
hypothesis,  it  is  not  an  impossible  one.  With  benzoic  acid  and 
water  we  get  two  liquid  phases  which  are  stable.  With  salicylic 
acid  and  water  the  two  liquid  phases  are  instable  but  can  be 
realized  without  any  difficulty.  On  this  basis  rapid  precipitation 
would  be  likely  to  get  us  down  to  the  instable  curve  for  the 
dineric  system  and  that  is  exactly  what  happens.  Von  Weimarn1 
obtains  any  anhydrous  inorganic  salt  as  a  gelatinous  mass  by 
precipitating  under  such  conditions  that  the  percentage  super- 
saturation  is  enormous.  It  is  also  interesting  to  note  that 
gelatinous  precipitates  are  obtained  only  as  a  result  of  condensa- 
tion methods.  Where  the  reverse  seems  to  be  the  case,  as  in  the 
peptization  of  silica,  it  is  probable  that  true  solution  occurs, 
followed  by  precipitation.  There  is  no  doubt  but  that  silicic  acid 
can  exist  temporarily  in  true  solution.  Mylius  and  Groschuff2 
showed  that  when  a  sodium  silicate  solution  was  acidified  with 
hydrochloric  acid,  the  silicic  acid  set  free  diffused  fairly  readily 
and  lowered  the  freezing-point.  These  two  characteristics  dis- 
appeared gradually  as  the  free  acid  changed  into  colloidal 
silica. 

If  one  objects  to  the  distinctly  arbitrary  assumption  that  we 
have  viscous  liquids  and  water  in  the  gelatinous  precipitates, 
one  alternative  is  to  assume  that  solid  particles  and  water  behave 
like  a  gelatinous  precipitate  when  the  solid  particles  are  suffici- 
ently fine  and  provided  they  adsorb  water  sufficiently  strongly. 

1  Grundziige  der  Dispersoidchemie,  67  (1914). 

2  Ber.  deutsch.  chem.  Ges.,  38,  116  (1905). 


GELATINOUS  PRECIPITATES  AND  JELLIES  239 

This  is  apparently  what  Zsigmondy1  does;  but  he  does  not  show 
why  this  should  be  so.  Another  alternative  is  to  assume  that 
the  adsorbed  ion  makes  the  surface  viscous.  In  the  case  of  an 
emulsion  this  does  happen.  We  may  have  drops  of  oil  coated 
with  a  soap  film  and  these  may  coalesce  sufficiently  to  form  a 
gelatinous  mass.  This  is  not  helpful  because  the  soap  is  gelat- 
inous in  itself.  It  is  possible,  however,  that  there  is  an  inter- 
mediate stage  between  that  of  peptization  and  that  of  irreversible 
coagulation,  where  there  may  be  a  surface  which  is  gelatinous 
in  its  properties.  While  something  of  this  sort  may  happen,  it 
has  not  been  shown  to  take  place.  The  real  test  would  be  to 
make  a  gelatinous  gold  precipitate  without  any  protecting  colloid. 
Until  something  of  this  sort  has  been  done,  or  until  we  know  why 
it  cannot  be  done,2  we  must  admit  that  we  know  very  little  in 
regard  to  what  constitutes  a  gelatinous  precipitate.  A  possible 
explanation  with  ferric  oxide  is  that  we  have  grains  of  oxide 
with  a  gelatinous  film  of  instable  ferric  hydroxide  adsorbed  on  the 
surface  and  stabilized  thereby.  This  would  not  be  inconsistent 
with  the  vapor  pressure  data  because  those  only  show  that 
ferric  hydroxide  does  not  exist  in  mass  under  the  conditions  of 
the  experiment.  We  know  that  sand  can  be  converted  into 
gelatinous  silicic  acid  if  ground  sufficiently  fine  and  that  clay 
particles  have  a  gelatinous  coating.  The  difficulty  is  that  this 
explanation  does  not  help  us  in  the  case  of  barium  sulphate  and 
we  must  either  explain  all  gelatinous  precipitates  in  the  same 
way  or  we  must  divide  them  into  groups  and  be  able  to  distinguish 
between  the  groups. 

We  do  not  know  for  certain  what  the  structure  of  a  gelatinous 
precipitate  is.  We  may  have  viscous  drops  coalescing  partly 
to  form  filaments  or  films,  or  we  may  have  spherical  drops  of 
water  coated  with  the  gelatinous  material.  This  second  case  is 
really  water  emulsified  in  water,  which  seems  improbable;  but 
a  mass  of  foam  is  air  emulsified  in  air,  so  that  such  a  system  is 
not  impossible.  This  would  constitute  a  honeycomb  structure 
whereas  the  other  type  gives  the  interlacing  or  sponge  structure, 
each  phase  being  continuous.  The  general  opinion  at  present 

1  Kolloidchemie,  149  (1912). 

2  It  has  been  suggested  that  gold  does  not  adsorb  water   sufficiently 
strongly  to  give  a  gelatinous  precipitate. 


240  APPLIED  COLLOID  CHEMISTRY 

is  that  gelatinous  precipitates  and  jellies  have  a  sponge  structure.1 
On  the  other  hand  we  know  that  certain  emulsion  jellies  have 
the  honeycomb  structure.  The  fact  that  one  can  do  ultra- 
filtration  through  a  suitable  gelatine  or  collodion  membrane  is  a 
strong  argument  in  favor  of  a  porous  structure  in  these  particular 
cases.  On  the  other  hand,  a  collodion  membrane  can  be  made 
which  is  less  permeable  to  air  and  more  permeable  to  hemoglobin 
than  the  collodion  ultra-filter,2  thus  indicating  an  entirely  differ- 
ent structure. 

We  can  now  consider  the  structure  of  jellies.  It  seems  pro- 
able  that  in  a  jelly  we  have  the  concentration  of  the  more  vis- 
cous constituent  equal  to  or  greater  than  that  necessary  to  give 
zero  fluidity.  It  has  already  been  shown  that  certain  graphite 
powders  give  zero  fluidity  at  a  concentration  of  5.5  .percent 
graphite  and  that  this  could  be  accounted  for  if  the  thickness  of 
the  adsorbed  film  were  a  little  greater  than  the  radius  of  the 
graphite  particle.  If  there  is  partial  agglomeration  to  larger 
masses,  the  viscosity  will  increase  and  zero  fluidity  can  be  reached 
at  lower  concentrations  for  the  same  relative  thickness  of  ad- 
sorbed film  or  at  the  same  concentration  with  a  lesser  relative 
thickness  of  adsorbed  film.  If  the  particles  coalesce  partially 
to  form  something  analogous  to  a  string  of  beads  or  if  they  form 
threads,  the  viscosity  will  be  increased  very  much  and  jellies 
can  be  obtained  at  very  low  concentrations.  All  these  things 
have  been  observed.  Bachmann3  has  shown  that  when  soap 
solutions  set  to  a  jelly,  long  threads  are  formed.  In  the  case  of 
sodium  stearate  a  translucent  jelly  can  be  obtained  containing 
less  than  one  percent  sodium  stearate.  These  threads  undoubt- 
edly go  over  finally  to  a  net  structure. 

With  starch  and  water  Meyer4  has  observed  the  formation  of 
a  net  structure  made  up  distinctly  of  drops  or  globules.  In 
cases  where  there  is  a  structure  without  the  visible  globules  being 
in  apparent  contact,  it  is  probable  that  the  larger  ones  are  ce- 

1  QUINCKE:  Drude'sAnn.,  10,  482  (1903);  14,  489  (1904);ZsiGMONDY:Zeit. 
anorg.  Chem.,  71,  356  (1911);  BACHMANN:  Ibid.,  73,  125  (1912);  FLADE: 
Ibid.,  82,  173  (1913). 

2  BECHHOLD:  Zeit.  phys.  Chem.,  64,  337  (1903). 

3  Zeit.  Kolloidchemie,  11,  145  (1912). 

4  Kolloidchemische  Beihefte,  5,  1  (1913). 


GELATINOUS  PRECIPITATES  AND  JELLIES  241 

merited  together  by  smaller  ones  which  are  invisible  under  the 
conditions  of  the  experiment.  It  is  interesting  to  note  that  the 
single  drops  are  never  homogeneous  but  are  always  turbid. 
With  gelatine  in  aqueous  alcohol  it  is  possible  to  get  a  net  structure 
made  up  of  globules  rather  than  filaments.  Meyer  considers 
that  all  the  starch-water  jellies  are  porous  masses  with  both 
phases  continuous.  Hardy1  believed  that  concentrated  gelatine 
jellies  consisted  of  drops  of  water  suspended  in  a  gelatine-rich 
phase;  but  this  conclusion  was  based  on  the  result  of  compressing 
jellies  and  consequently  needs  further  confirmation. 

It  seems  probable  that  the  concentrated  gelatine  jellies  with 
drops  of  water  suspended  in  a  gelatine-rich  phase  should  not  be 
considered  to  have  a  honeycomb  structure  in  the  sense  that  an 
emulsion  has  a  honeycomb  structure.  With  the  emulsion  there 
is  a  film  of  a  third  substance  around  the  dispersed  drops  and  we 
should  still  have  an  emulsion  if  the  outer  phase  could  suddenly 
be  made  fluid.  Under  similar  conditions  the  drops  of  water  in 
a  gelatine  jelly  would  undoubtedly  run  together.  A  gelatine 
jelly  of  the  type  under  consideration  is  merely  a  viscous  medium 
in  which  water  is  dispersed.  If  we  knead  water  into  butter  we 
do  not  get  a  true  emulsion  any  more  than  we  get  a  true  froth 
when  we  knead  air  into  dough  or  into  soap.  In  the  case  of 
emulsions  the  type  depends  on  the  nature  of  the  third  substance 
and  not  on  the  relative  masses  of  the  two  liquids.  If  a  gelatine- 
water  mixture  were  the  water-in-oil  type  at  any  concentration, 
it  would  have  to  be  a  water-in-oil  type  at  all  concentrations.  A 
good  deal  of  stress  has  been  laid  at  times  on  the  alleged  fact  that 
ultramicroscopic  examination  has  shown  that  jellies  have  a 
sponge  structure  and  not  a  honeycomb  structure;  but  it  is  at 
last  admitted  that  we  cannot  distinguish  between  these  two 
cases  by  means  of  the  ultra-microscope.2 

It  has  always  been  assumed  tacitly  that  each  so-called  phase 
in  a  gelatine  jelly  is  truly  homogeneous  and  attempts  have  been 
made  to  determine  the  composition  of  one  of  the  phases  by 
squeezing  liquid  out  of  a  gelatine  jelly  and  analyzing  it.  Quite 
apart  from  errors  of  manipulation,  which  are  probably  large, 
the  underlying  assumption  is  not  necessarily  sound.  If  we 

1  Zeit.  phys.  Chem.,  33,  326  (1900). 

2  BACHMANN:  Zeit.  Kolloidchemie.,  23,  89  (1918). 

16 


242  APPLIED  COLLOID  CHEMISTRY 

shake  up  fairly  pure  oil  with  fairly  pure  water,  the  mixture 
separates  into  two  liquid  layers,  the  oil  layer  containing  some 
dispersed  water  and  the  water  layer  containing  some  dispersed 
oil.  Since  water  peptizes  gelatine  under  certain  conditions, 
there  is  no  reason  liquid  gelatine  or  a  gelatine-rich  phase  should 
not  peptize  water.  It  is  probable  therefore  that  with  gelatinous 
precipitates  and  with  jellies  we  have  a  binary  system  in  which 
each  constituent  peptizes  the  other.  A  necessary  characteristic 
of  such  a  system  is  that  what  we  ordinarily  consider  the  separate 
phases  will  never  be  homogeneous.  The  gelatine-rich  phase  will 
always  contain  peptized  water  and  the  water-rich  phase  will 
always  contain  peptized  gelatine.  Theoretically  this  can  be 
carried  on  indefinitely  because  the  peptized  water  may  peptize 
gelatine  and  the  peptized  gelatine  may  peptize  water;  but  practi- 
cally there  will  soon  come  a  point  beyond  which  no  effect  can  be 
detected.  The  predicted  lack  of  homogeneity  in  what  would 
ordinarily  be  a  separate  phase  has  been  commented  on  by  Meyer1 
in  his  studies  on  starch. 

A  dilute  colloidal  solution  of  gelatine  in  water  will  consist 
essentially  of  turbid  drops  of  a  gelatine-rich  phase  dispersed  in 
water.  With  increasing  concentration  of  gelatine,  there  will  be 
a  tendency  for  the  separate  drops  to  coalesce  into  larger  drops. 
If  they  are  too  viscous  to  do  this,  they  may  coalesce  partially 
forming  threads  or,  to  a  still  lesser  extent,  forming  a  chain  of 
beads.  In  this  latter  case  it  is  not  necessary  that  the  drops 
themselves  should  be  in  actual  contact.  In  the  preliminary 
stages  it  may  only  be  the  adsorbed  films  of  water  which  are  in 
contact,  just  as  happens  with  a  plastic  mass  of  sand  and  water. 
If  the  dispersed  drops  are  of  different  sizes,  the  smaller  drops 
will  probably  group  themselves  around  the  larger  ones,  so  that 
the  latter  will  be  held  in  place  by  a  much  less  visible  group  of 
smaller  drops. 

With  increasing  concentration  of  gelatine,  loose  chains  may 
be  formed  which  will  then  pass  into  a  net  or  sponge  structure  of 
which  a  granular  jelly  is  an  extreme  form.  In  jellies  of  this  type, 
both  phases  are  continuous  and  we  are  dealing  with  an  interlacing 
system  With  still  further  increase  in  the  gelatine  we  may  get 
water  dispersed  as  definite  drops  in  the  gelatine-rich  phase. 

1  Kolloidchemische  Beihefte,  6,  1  (1913). 


GELATINOUS  PRECIPITATES  AND  JELLIES  243 

While  this  might  be  called  a  honeycomb  structure,  Meyer1  is 
probably  right  in  not  so  classifying  it.  It  is  probably  better  to 
reserve  the  term  "honeycomb"  for  structures  in  which  the  cell 
walls  are  thin  relatively  to  the  diameter  of  the  cells  and  it  may 
be  desirable  to  restrict  it  to  cases  like  the  emulsions  where  there 
is  a  distinct  film  round  the  drops. 

Owing  to  our  comparative  ignorance  in  regard  to  jellies,  we  do 
not  know  to  what  extent  increasing  the  concentration  of  gelatine, 
for  instance,  causes  the  mass  to  fill  up  with  chains  or  filaments 
of  the  same  diameter  or  to  what  extent  each  filament  increases  in 
cross-section.  It  is  the  absence  of  a  satisfactory  working  hypoth- 
esis that  makes  it  so  difficult  to  account  for  many  of  the 
properties  of  jellies. 

If  a  porous  jelly  of  the  interlacing  type  be  allowed  to  dry  at 
constant  temperature,  the  walls  may  be  rigid  enough  not  to 
collapse  completely,  in  which  case  we  shall  eventually  get  an 
interlacing  system  with  air  in  the  pores.  If  the  walls  are  not 
rigid,  they  may  collapse  to  such  an  extent  that  the  pores  will 
disappear  practically  completely.  If  the  walls  do  not  unite 
when  thus  brought  in  contact,  or  if  there  is  formed  something 
analogous  to  a  scar,  the  behavior  of  the  jelly,  when  placed  in 
water,  will  vary  with  its  previous  history.  If  the  walls  coalesce 
absolutely  and  uniformly,  the  rate  and  amount  of  swelling  will  be 
independent  of  the  previous  history  of  the  jelly. 

Since  a  gelatinous  precipitate  differs  from  a  jelly  jn^that  the 
latter  has  not  contracted  and  excreted  liquid,  it  follows  that  we 
shall  get  jellies  when  the  conditions  for  precipitation  are  such  as 
to  avoid  marked  contraction.  With  gelatine  or  agar  the  tend- 
ency to  fonn  jellies  is  so  strong  that  no  special  precautions  are 
necessary.  In  most  cases,  however,  a  slow  precipitation  is 
essential  where  there  is  a  tendency  to  form  a  gelatinous  precipi- 
tate.2 A  striking  instance  of  this  occurs  in  dialysis  where  the 
liquid  in  the  dialyzer  is  very  apt  to  set  to  a  jelly  just  before  the 
colloid  is  entirely  pure,  in  other  words  just  as  the  peptizing  agent 
is  brought  slowly  below  a  critical  value.3  Slightly  sour  milk 
coagulates  on  boiling  and  a  very  acid  milk  will  set  to  a  solid  mass 

1  Kolloidchemische  Beihefte,  6,  30  (1913). 

2  HOLMES  and  FALL:  Jour.  Am.  Chem.  Soc.,  41,  763  (1919). 

3  HOLMES  and  ARNOLD:  Jour.  Am.  Chem.  Soc.,  40,  1014  (1918). 


244  APPLIED  COLLOID  CHEMISTRY 

at  room  temperature.  This  is  a  case  of  a  gradual  production  of 
precipitating  agent  and  therefore  fits  in  with  our  general  point 
of  view.  This  is  not  always  understood  and  it  is  a  popular 
belief  in  Sweden  that  theso-called"dicke  Milch"  cannot  bemade 
in  winter.  All  this  means  is  that  the  kitchens  are  ordinarily 
so  cold  in  winter  that  the  lactic  acid  fermentation  does  not  take 
place  sufficiently  rapidly.  The  same  effect  can  be  produced 
by  a  suitable  addition  of  acetic  acid.1  A  so-called  calcium  suc- 
rate  solution  sets  to  a  jelly  on  addition  of  carbon  dioxide.2 
When  caseine  is  heated  with  water,  the  solution  becomes  more 
turbid  and  this  turbidity  is  more  marked  in  presence  of  calcium 
salts  which  tend  to  precipitate  it.3  With  a  suitable  amount  of 
calcium  chloride  the  liquid  clots.4  Calcium  chloride  and  milk 
require  a  higher  temperature  to  cause  clotting  and  calcium  chlo- 
ride does  not  cause  milk  to  set  to  a  jelly  at  ordinary  temperatures. 
It  is  not  known  what  constituent  in  the  milk  checks  the  clotting. 
There  are  a  number  of  things  about  the  coagulation  of  milk  by 
rennin  which  need  further  study. 

In  the  case  of  some  inorganic  jellies,  the  presence  of  certain 
ions  seems  to  be  necessary  either  to  ensure  sufficiently  slow  pre- 
cipitation or  to  prevent  contraction.  To  get  chromic  oxide 
jellies5  acetate  or  sulphate  must  be  present.  With  cupric  oxide 
jellies6  a  small  amount  of  sulphate  is  necessary.  It  is  surprising 
in  what  a  number  of  cases  it  is  possible  to  get  inorganic  jellies. 
Delafontaine7  found  that  jellies  are  formed  when  ammonia  is 
added  to  yttrium,  didymium,  lanthanum,  and  erbium  acetates. 
Ferric  oxide,  glycerine  and  alkali  form  a  jelly  on  dialysis  which 
liquefies  on  adding  alkali.8  Zinc  oxide9  and  mercuric  oxide10 


:  Zeit.  phys.  Chem.,  41,  189  (1904). 

2  HORSIN-DEON:  Jour.  Chem.  Soc.,  26,  612  (1872). 

3  VAN  SLYKE  and  HART:  Am.  Chem.  Jour.,  33,  467  (1905). 
*  RINGER:  Jour.  Physiology,  11,  470  (1890). 

BBUNCE  and  FINCH:  Jour.  Phys.  Chem.,  17,  269  (1913);  NAGEL:  Ibid., 
19,  331  (1915). 

6  FINCH:  Jour.  Phys.  Chem.,  18,  26  (1914). 

7  Chem.  News,  73,  284  (1896). 

8GRiMAux:  Comptes  rendus,  98,  1485,  1540  (1884);  FISCHER:  Biochem. 
Zeit.,  27,  223  (1910). 

9KuRiLOFr:  Zeit.  Elektrochemie,  12,  213  (1906). 
10  BUNCE:  Jour.  Phys.  Chem.,  18,  269  (1914), 


GELATINOUS  PRECIPITATES  AND  JELLIES  245 

jellies  have  been  made.  Klemp  and  Gyrlai1  discuss  the  forma- 
tion of  ar senate  jellies.  Crum2  made  an  alumina  jelly  con- 
taining 1/600  alumina  and  1/7500  sulphuric  acid.  Bentley  and 
Rose3  dissolve  hydrous  alumina  in  8  percent  acetic  acid  and 
add  acetic  acid.  Pickering4  states  that  basic  aluminum  sul- 
phate can  be  obtained  either  flocculent  or  as  a  jelly.  It  is  a 
little  difficult  to  see  how  a  jelly  containing  1/600  alumina  and 
1/7500  sulphuric  acid  can  possibly  be  solid  if  it  has  a  sponge  or 
net  structure,  whereas  there  would  be  no  difficulty  in  case  there 
was  a  honeycomb  structure.  It  is  also  not  clear  how  coagulation 
to  a  net  structure  can  give  a  gel  without  any  supernatant  liquid. 
There  is  a  good  deal  more  careful  work  to  be  done  on  jellies 
before  we  can  be  sure  of  what  happens  in  each  particular  case. 
With  one  percent  sodium  stearate  jellies,  it  seems  to  be  certain 
that  the  particles  agglomerate  to  form  filaments.5 

VAPOR  PRESSURE  RELATIONS 

If  the  inorganic  jellies  are  not  hydrated  we  should  expect  the 
vapor  pressure  concentration  curve  to  be  similar  in  type  with 
that  of  charcoal  and  water,  a  smooth  curve  with  no  breaks. 
This  is  what  happens  with  the  exception  that  the  pores  in  the 
jellies  may  change  in  size,  in  which  case  the  curve  will  not  be 
reversible. 

Over  fifty  years  ago,  Davies6  showed  that  hydrous  ferric  oxide 
could  be  dehydrated  completely  by  prolonged  heating  under 
water,  which  of  course  could  not  be  the  case  if  a  definite  hydrate 
were  formed;  but  is  just  what  might  be  expected  if  the  ferric  oxide 
changed  in  structure  and  became  less  adsorbent.  Over  forty 
years  ago  Ramsay7  showed  that  the  rate  at  which  hydrous  ferric 
oxide  lost  water  was  not  constant  and  that  consequently  there 
could  be  no  definite  hydrate  within  the  range  studied.  Twenty 

1  Zeit.  Kolloidchemie,  16,  202  (1914). 

2  Jour.  Chem.  Soc.,  6,  216  (1854). 

3  Jour.  Am.  Chem.  Soc.,  35,  1490  (1913);  ROSE:  Zeit.  Kolloidchemie,  16, 
1  (1914). 

4  Jour.  Chem.  Soc.,  42,  700  (1882). 
6BACHMANN:  Zeit.  Kolloidchemie,  11,  145  (1912). 

6  Jour.  Chem.  Soc.,  19,  71  (1865). 

7  Ibid.,  32,395  (1877). 


246  APPLIED  COLLOID  CHEMISTRY 

years  ago  van  Bemmelen1  determined  the  isotherm  for  ferric 
oxide  and  water,  finding  no  indication  of  a  definite  compound. 
On  the  other  hand,  a  definite  compound,  Fe2O3.H2O,  is  found  in 
nature  as  the  minerals,  Gothite  and  Lepidocrite.2  This  can  be 
made  in  the  laboratory  by  decomposing  sodium  ferrite,  Na2FeO4, 
but  in  no  other  way.  This  hydrate  behaves  on  dehydration 
exactly  as  a  definite  compound  should.  The  vapor  pressure 
of  the  hydrous  ferric  oxide  changes  with  the  time  and  therefore 
depends  on  the  previous  history  of  the  sample. 

There  is  no  evidence  of  any  hydrate  of  chromic  oxide3  between 
15°  and  28°.  The  case  of  hydrous  cupric  oxide  is  a  little  more 
difficult  because  the  vapor  pressure  becomes  practically  zero 
at  the  composition,  CuO.H2O,  and  consequently  one  cannot  tell 
from  pressure  measurements  whether  a  definite  compound  occurs 
or  not.  If  a  definite  compound  existed  with  a  practically  zero 
pressure,  it  should  form  from  cupric  oxide  in  presence  of  water. 
Actually  the  reverse  thing  happens  and  the  blue  gelatinous 
cupric  oxide  goes  over  spontaneously  into  the  black  cupric  oxide 
under  water.  This  change  can  be  prevented  by  the  presence  of 
hydrous  oxides  of  nickel,  cobalt,  chromium,  aluminum,  mercury, 
magnesium,  zinc,  or  manganese.4  Consequently  we  conclude 
that  no  definite  hydrate  is  formed  under  ordinary  conditions. 
This  does  not  exclude  the  possibility  of  a  definite  hydrate5  being 
formed  under  special  conditions. 

While  these  experiments  show  that  some  cherished  hydroxides 
do  not  exist,  there  are  others  which  stand  the  test.  Van  Bemme- 
len recognizes  the  existence  of  BeO.H^O,  MgO.H2O,  MnO.H2O, 
and  A12O3.3H2O.  It  is  not  clear  why  A12O3.3H2O  should  be  a 
definite  compound  and  Fe2O3.3H2O  and  Cr2O3.3H2O  not;  but 
that  seems  to  be  the  case. 

The  case  of  silicic  acid  has  been  studied  with  great  care  by 
van  Bemmelen.6  A  dilute  jelly  may  contain  about  300  mols  of 
water  to  one  of  silica.  Such  a  jelly  flows  together  again  when 

1  Zeit.  anorg.  Chem.,  20,  185  (1899). 

2  POSNJAK  and  MERWIN:  Am.  Jour.  Sci.  (4),  47,  311  (1919). 

3  VAN  BEMMELEN:  Die  Absorption,  78  (1910). 

4  BLUCHER  and  FARNATJ:  Jour.  Phys.  Chem.,  18,  629  (1914). 
6  BONSDORFF:  Zeit.  anorg.  Chem.,  41,  134  (1904). 

6  Die  Absorption,  196,  214,  232  (1910). 


GELATINOUS  PRECIPITATES  AND  JELLIES 


247 


broken  into  pieces.  When  the  water  content  is  only  30-40  mols 
the  gel  can  be  cut  and  will  stand  alone.  With  20  mols  the  gel 
is  stiff,  and  it  is  brittle  when  the  content  is  10  mols.  At  6  mols 
the  gel  can  be  pulverized  and  the  powder  is  apparently  dry. 
The  vapor  pressure  is  practically  that  of  pure  water  down  to  a 
water  content  of  6  mols  per  mol  of  silica.  On  further  dehydra- 
tion the  vapor  pressure  curve  drops  continuously  with  no  sign  of 
the  existence  of  any  definite  compound.  When  the  water  con- 
tent drops  to  somewhere  usually  between  1.5  and  3.0  mols  with 
different  samples,  the  gel  becomes  opaque  and  chalky  but  clears 
up  again  when  the  water  content  reaches  about  0.5-1.0  mols. 
The  cloudiness  begins  when  the  first  air  bubbles  appear  in  the 
pores  and  lasts  until  the  pores  are  filled  completely  with  air. 
Owing  to  the  action  of  capillarity  the  water  which  evaporates 
from  the  outer  surface  of  the  capillaries  is  at  once  replaced  from 
the  inside  of  the  gel,  leaving  a  vapor  space  in  the  centre  of  the 
j  elly .  This  produces  an  opacity  which  lasts  until  the  pores  are 
free  from  capillary  water.  The  balance  of  the  water  0.5  -1.0  mol 
is  probably  adsorbed  so  strongly  on  the  surface  of  the  capillaries 
that  it  is  almost  impossible  to  remove  it,  just  as  we  have  already 
seen  that  heating  charcoal  100°  above  the  boiling-point  of  ben- 
zene does  not  necessarily  remove  the  last  traces  of  benzene. 
As  the  water  passes  off,  the  volume  of  the  gel  shrinks  until  the 
clouding  begins,  after  which  there  is  practically  no  change  in 
volume  during  further  loss  of  water.  This  is  shown  by  the 
data  in  Table  XL VII. 


TABLE  XLVII. — VOLUMES  OP  SILICA  JELLIES 


Mols  H20 
per  mol 
Si02 

Volume 

Mols  H20 
per  mol 
Si02 

Volume 

Mols  HiO 
per  mol 
SiO2 

Volume 

122.0 
75.7 

29 

18 

11.3 

2.8 

3.0 
1.0 

clouding 
1.0 

0.73 

45.2 

11 

2.2 

0.86 

0.39 

0.73 

23.2 

4 

1.7 

0.75 

0.3  t 

0.73 

In  Fig.  10  is  given  a  schematic  representation  of  the  pressure- 
concentratiori  relations  at  15°  for  a  freshly  prepared  silica  gel. 


248 


APPLIED  COLLOID  CHEMISTRY 


The  A-curves  represent  the  first  dehydration  over  sulphuric 
acid.  The  Z-curves  are  for  material  which  has  been  dehydrated 
once  more  or  less  completely.  The  direction  of  the  arrows  shows 
whether  dehydration  is  taking  place.  Starting  with  a  freshly 
prepared  gel,  the  vapor  pressure  drops  below  that  for  pure  water 
and  the  dehydration  curve  consists  of  the  three  portions  Aft 
Aa/3  and  Aa.  There  is  probably  an  actual  break  in  the  curve 
at  O,  where  the  gel  begins  to  cloud  and  it  is  doubtful  whether 
there  is  a  break  at  Oi  where  the  clouding  ends.  Owing  to  the 
decrease  in  volume  as  far  as  O,  the  capillaries  are  entirely  filled 
with  water  and  the  gel  is  transparent.  Since  the  volume  does 
not  change  much  after  reaching  0,  the  capillaries  necessarily 


S/O2 


FlG.    10. 

begin  to  fill  with  air  as  the  water  evaporates  and  the  gel  becomes 
turbid  even  up  to  the  point  of  chalkiness.  Owing  to  capillary 
action  the  liquid  is  always  drawn  to  the  outer  surface  of  the  capil- 
laries and  the  clouding  starts  at  the  center.  By  the  time  Oi  is 
reached  the  capillaries  are  full  of  air  except  for  a  layer  of  strongly 
adsorbed  water  and  the  gel  becomes  clear  again.  Along  the 
curve  Aa  the  last  traces  of  adsorbed  water  are  driven  off.  If 
the  dehydration  is  stopped  at  some  point  on  A/8  and  if  the  gel  is 
exposed  to  higher  partial  pressure  of  water  vapor,  it  is  found 
that  the  hydration  is  not  reversible  and  we  get  a  curve  Zy. 
While  the  gel  shrinks  along  Aft  it  does  not  swell  again  to 
any  appreciable  extent  and  consequently  it  does  not  take 
up  water  under  the  same  conditions.  The  curve  Zy  repre- 


GELATINOUS  PRECIPITATES  AND  JELLIES  249 

sents  reversible  phenomena  at  least  until  the  dehydration  has 
been  carried  beyond  the  point  where  Zy  meets  A/8.  From  O  to  Oi 
we  have  an  irreversible  pressure  change.  If  the  dehydration  is 
stopped  at  any  point  we  get  curves  for  hydration  like  Z0  and  Z5 
which  usually  meet  as  drawn  in  the  point  O2.  From  O2  to  Oa 
the  pressure-concentration  curve  is  reversible.  From  O%  we 
pass  to  0  on  dehydration.  It  is  possible  to  pass  along  the  path 
OOiO2O  as  often  as  one  likes  but  only  in  the  one  direction. 
The  loop  OOiO2O  is  therefore  a  hysteresis  loop.  Zsigmondy1 
considers  that  this  is  due  to  adsorbed  air  preventing  the  water 
from  wetting  the  capillaries  readily  but  this  can  hardly  be  so 
because  the  capillaries  have  not  dried  out.  It  seems  more  prob- 
able that  the  water  condenses  first  at  the  outside  of  the  capil- 
laries and  that  the  hysteresis  is  due  to  the  entrapped  air  or  vapor. 
On  this  hypothesis  the  hysteresis  should  be  less  marked  if  the 
hydration  and  dehydration  were  carried  on  in  the  absence  of  air. 
This  experiment  has  been  tried2  but  without  any  striking  results. 
The  break  at  O  was  much  less  marked  and  the  hysteresis  loop 
was  somewhat  narrower;  but  it  is  not  certain  that  this  may  not 
be  due  to  differences  in  the  preparation  of  the  gel.  Conse- 
quently there  is  no  conclusive  proof  of  the  cause  of  the  hysteresis. 
The  matter  is  complicated  further  by  the  fact  that  O2  does  not 
always  lie  on  the  direct  line  from  Os  to  O;  but  may  sometimes 
be  to  the  left  of  it.  From  the  observed  vapor  pressures  Anderson3 
calculates  that  the  diameter  of  the  pores  is  about  2.4-55/i/z.  It 
has  been  suggested  that  OOi  would  be  a  flat  if  all  the  pores  were 
of  the  same  size.  The  water  removed  along  OiOo  is  probably 
the  last  film  of  water  adsorbed  on  or  in  the  capillaries.  Zsig- 
mondy considers  that  it  may  be  water  in  solid  solution  in  the 
silica;  but  this  seems  improbable  because  in  that  case  this  portion 
of  the  curve  should  be  independent  of  the  history  of  the  sample 
which  is  not  so,  though  there  is  nothing  impossible  about  water 
forming  a  solid  solution  with  quartz.  Tammann*  has  shown 
that  magnesium  platinocyanide  and  zeolites  do  not  lose  their 

1  Kolloidchemie,  161  (1912).     Spear's  translation  of  this  is  inaccurate. 

2  ZSIGMONDY,  BACHMANN  and  STEVENSON:  Zeit.  anorg.  Chem.,  76,  189 
(1912). 

3  Zeit.  phys.  Chem.,  88,  191  (1914). 

4  Wied.  Ann.,  63,  16  (1897);  Zeit.  phys.  Chem.,  20,  323  (1898). 


250  APPLIED  COLLOID  CHEMISTRY 

water  of  crystallization  at  constant  pressure  and  do  not  cloud. 
He  assumes  consequently  that  the  water  is  present  as  a  solid 
solution.  This  has  been  confirmed  by  Lowenstein1  who  found  a 
similar  behavior  with  strychnine  sulphate  and  with  the  oxalates 
(but  not  the  nitrates  or  sulphates)  of  cerium,  lanthanum,  erbium, 
yttrium,  thorium,  and  zirconium.  In  these  cases  there  is  no 
evidence  that  the  vapor  pressures  depend  on  the  previous  histoiy 
of  the  sample. 

If  a  silicic  acid  gel  is  heated,  it  sinters  and  contracts,  thereby 
making  the  pores  finer.  They  consequently  do  not  hold  so 
much  water  and  owing  to  the  change  in  structure  they  do  not 
hold  it  so  firmly.  Consequently  the  whole  diagram  is  moved  to 
the  left  and  the  curve  OOi  becomes  much  shorter.  If  the  silicic 
acid  gel  is  allowed  to  stand  for  a  long  time  at  room  temperature, 
there  is  a  change  in  the  walls  of  the  capillaries  such  that  the 
contraction  of  the  gel  on  dehydration  ceases  at  an  earlier  point. 
Under  these  circumstances  the  curve  OOi  becomes  longer  be- 
cause the  pores  are  coarser  when  they  begin  to  empty.  Since 
there  is  less  water  adsorbed  strongly  by  the  capillaries 
more  water  has  to  be  removed  before  the  pores  are  entirely 
empty. 

That  the  curve  OOi  is  not  due  to  the  presence  of  a  definite 
hydrate  is  shown  by  the  fact  that  the  position  and  length  of  this 
curve  are  so  variable  and  by  the  fact  that  similar  optical  phenom- 
ena can  be  observed  when  alcohol  or  benzene  is  substituted  for 
water  in  the  jelly.  With  alcohol  two  differences  are  noted  in  the 
pressure-concentration  curve.2  The  break  at  O  is  not  so 
marked  and  the  point  O2  is  far  to  the  left  of  O,  almost  down  to 
Oi.  The  significance  of  these  two  variations  was  not  worked 
out  by  Anderson. 

Holmes,  Kaufmann  and  Nicholas3  have  shown  that  when  a 
suitable  silicic  acid  jelly  is  allowed  to  form  in  a  glass  test-tube, 
it  vibrates  like  a  rigid  solid  when  tapped.  Some  of  these  gels 
gave  a  tone  two  octaves  above  middle  C.  The  vibration  frequency 
varies  directly  as  the  concentration  of  silicic  acid,  increases  with 
excess  of  mineral  acids,  and  decreases  with  excess  of  organic  acid. 

1  Zeit.  anorg.  Chem.,  68,  69  (1909). 

2  ANDERSON:  Zeit.  phys.  Chem.,  88,  191  (1914). 

3  Jour,  Am.  Chem.,  41,  1329  (1919). 


GELATINOUS  PRECIPITATES  AND  JELLIES  251 

The  pitch  does  not  vary  with  the  length  of  the  column  of  gel 
alter  a  minimum  length  is  exceeded.  With  gel  columns  60,  90, 
and  120  mm  in  length  and  all  17  mm  in  diameter,  the  frequency 
was  1024  per  second  in  each  case,  indicating  that  the  vibrations 
are  not  longitudinal.  With  columns  of  the  same  length  but  of 
different  diameters,  the  frequencies  were  341, 640, 1024,  and  1152 
per  second  when  the  diameters  were  34,  23,  17,  and  15  mm 
respectively.  The  vibration  frequency  varies  approximately 
inversely  as  the  diameter  and  consequently  the  vibrations  are 
transverse.  When  the  gels  were  formed  in  vaselined  tubes,  they 
did  not  adhere  to  the  walls  and  the  vibration  frequency  was  lower 
than  for  similar  gels  adhering  to  the  glass  walls.  This  shows 
that  under  ordinary  circumstances  the  silica  jellies  are  in  a  state 
of  tension  when  formed  in  glass  vessels  and  that  this  tension 
has  an  effect  on  the  pitch  of  the  tone  which  they  emit  when 
tapped. 

It  has  been  shown  that  a  silica  gel  containing  a  great  deal  of 
water  shrinks  very  much  when  the  water  is  removed  down  to  a 
certain  point;  but  the  volume  change  is  not  reversible.  With  a 
gelatine  jelly  the  change  is  more  nearly  reversible.  If  dried 
gelatine  is  placed  in  cold  water  it  swells  a  good  deal  and  may  take 
up  ten  times  its  weight  of  water;  but  there  are  no  experiments 
to  show  that  it  would  ever  go  up,  say  to  an  eight  percent  jelly. 
On  the  other  hand  it  is  possible  to  start  with  an  eight  percent 
jelly  and  dry  it  to  a  ninety-six  percent  jelly,  after  which  it 
will  take  up  water  rapidly  to  an  eight  percent  jelly.  This  means 
that  the  structure  of  the  gelatine  plays  an  important  part  in  the 
rate  of  swelling.  This  is  confirmed  by  some  unpublished  pre- 
liminary results  by  Mr.  Cartledge.  Gelatine  jellies  were  made 
up  coritaining  8,  16,  24  and  32  percent  of  gelatine.  These 
were  all  dried  at  room  temperature  to  about  96  percent  concen- 
tration. When  water  was  added,  each  swelled  rapidly  to  the 
original  concentration  and  then  took  up  water  slowly.  If  these 
results  are  accurate,  it  means  that  the  four  96  percent  jellies 
were  all  different  and  that  the  8  percent  gelatine  did  not  become 
like  the  16  percent,  24  percent  or  32  percent  gelatines  while 
being  dried.  This  is  very  important  if  true,  because  this  possi- 
bility is  usually  overlooked.  Arisz1  finds  that  at  20°  a  half 

1  Kolloidchemische  Beihefte,  7,  1  (1915). 


252  APPLIED  COLLOID  CHEMISTRY 

percent  gelatine  jelly  disintegrates  completely  in  water,  a  ten 
percent  jelly  goes  to  a  two  percent  jelly  in  four  days,  a  twenty 
percent  jelly  to  a  six  percent  one,  a  fifty  percent  jelly  to  a  sixteen 
percent  one,  and  an  eighty  percent  jelly  to  a  twenty  percent 
one.  Although  the  twenty  percent  jelly  will  take  up  enough 
water  so  that  its  composition  is  equal  to  that  of  a  ten  percent 
jelly,  the  two  do  not  then  behave  alike.  The  jelly  which  has 
swelled  until  its  composition  is  ten  percent  will  not  then  take  up 
as  much  water  in  a  reasonable  time  as  a  jelly  which  is  made  up 
at  ten  percent.  This  shows  that  there  is  probably  a  difference 
in  structure  and  in  the  way  in  which  the  water  is  held,  although 
Sheppard  considers  that  these  phenomena  are  due  to  changes 
in  the  shape  of  the  mass  and  to  a  consequent  unequal  distri- 
bution of  water.  Until  we  have  some  satisfactory  theory 
to  account  for  this  difference  in  behavior,  all  experiments  on  the 
swelling  of  gelatine  jellies  in  solutions  of  salts,  acids  and  bases 
are  likely  to  be  misleading  because  one  does  not  know  to  what 
extent  the  electrolytes  are  causing  a  change  in  structure.  That  a 
change  in  structure  takes  place  even  with  water  is  shown  by  the 
experiments  of  Arisz  on  intermittent  soaking.  If  two  identical 
gelatine  jellies  are  placed  in  water  and  one  is  kept  in  the  water 
six  days,  while  the  other  is  in  the  water  only  on  the  first,  fifth, 
and  sixth  days,  the  amount  of  swelling  will  be  the  same  in  the 
two  cases  within  the  limits  of  experimental  error.  While  the 
partially  swollen  jelly  is  out  of  the  water,  some  change  in  struc- 
ture takes  place  such  that  it  takes  up  water  so  much  more  rapidly 
than  the  other  that  the  total  swelling  is  the  same  in  the  two 
cases,  although  one  jelly  was  soaked  twice  as  long  as  the  other. 
This  seems  to  be  a  case  without  a  moral  because  the  hare  caught 
up  with  the  tortoise.  Mr.  C.  R.  Smith  tells  me  that  ash-free 
gelatine  does  not  set  to  a  jelly  in  cold  water  but  comes  out  as  a 
precipitate.  This  fact  must  be  kept  in  mind  in  any  discussion 
of  the  theory  of  the  swelling  of  gelatine. 

In  certain  salt  solutions  gelatine  swells  more  than  in  water. 
With  sodium  salts  the  swelling  decreases  in  the  order  CNS  >  I  > 
Br  >  NO3  >  C1O3  >  Cl  >  acetate  > citrate  >tartrate>SO4.  The 
last  four  cause  a  lesser  swelling  than  in  pure  water.  At  a  con- 
centration of  13.8  percent  NaCl  the  amount  of  water  taken  up 
passes  through  a  maximum.  Calcium  chloride  causes  more  swell- 


GELATINOUS  PRECIPITATES  AND  JELLIES  253 

ing  than  sodium  chloride. 1  Gelatine  swells  more  in  dilute  acid  and 
dilute  alkali  solutions  than  in  water  or  in  salt  solutions,  the  order 
for  the  acids  being  HC1>HNO3>CH3CO2H>H2SO4>H3BO3. 
With  the  strong  acids  the  swelling  passes  through  a  maximum 
for  a  given  concentration  and  then  decreases.  Salts  cut  down  the 
swelling  both  in  hydrochloric  acid  and  in  caustic  soda  solutions, 
the  order  being  the  same  in  both  cases.  The  chlorides,  bromides, 
and  nitrates  have  less  effect  in  cutting  down  the  swelling  than  do 
the  sulphates  and  acetates.  There  is  said  to  be  no  reversal  of 
anions  when  one  changes  from  an  acid  to  an  alkaline  solution.2 
Fibrine  behaves  very  much  like  gelatine  though  the  amount  of 
swelling  is  different  and  the  order  of  the  anions  is  not  absolutely 
identical.  Although  a  great  deal  of  work  has  been  done  on  the 
swelling  of  gelatine  and  fibrine3  and  some  work  on  the  adsorption 
by  fibrine4  and  by  gelatine,5  very  little  has  been  accomplished 
along  the  line  of  developing  a  satisfactory  theory.  If  we  pos- 
tulate that  an  adsorbed  anion  is  the  important  factor,  as  seems 
to  be  the  case  with  the  salts,  then  the  amounts  of  any  two  anions 
adsorbed  must  reverse  as  one  changes  from  an  acid  to  an  alkaline 
solution.  If  an  important  factor  is  the  adsorption  of  an  undis- 
sociated  salt,  sodium  and  potassium  salts  should  behave  quite 
differently.  It  is  probable  that  part  of  the  difficulty  is  in  the 
way  that  the  experiments  have  been  carried  out.  With  fibrine, 
for  instance,  there  is  a  maximum  swelling  at  less  than  N/10  for 
hydrochloric  acid,  nitric  acid,  and  sulphuric  acid,  whereas  with 
acetic  acid  there  is  no  sign  of  a  maximum  even  with  normal 
acid.  If  we  compare  dilute  acids,  say  0.02N,  nitric  acid  and 
hydrochloric  acid  cause  more  swelling  than  acetic  acid;  but  with 
normal  acid  the  swelling  is  much  greater  in  the  case  of  acetic 
acid.6  When  sodium  chloride  is  added  to  0.015N  solutions  of 

1  This  must  refer  to  a  slightly  acid  gelatine.    In  none  of  the  work  has 
the    hydrogen    ion    concentration    been   controlled   sufficiently   carefully. 
Loeb  has  shown  the  importance  of  this;  but  his  work,  so  far,  has  been 
with  such  dilute  salt  solutions  that  he  did  not  detect  any  adsorption  of 
the  salt  cation  in  acid  solutions  or  of  the  salt  anion  in  alkaline  solutions. 

2  FISCHER:  Das  Oedem,  37  (1910). 

3  FISCHER:  Jour.  Am.  Chem.  Soc.,  40,  272,  292,  303  (1918). 

4  TOLMAN  and  STEARN:  Ibid.,  40,  264  (1918). 

5  DUMANSKI:  Zeit.  phys.  Chem.,  60,  553  (1907). 

6  TOLMAN  and  STEARN:  Jour.  Am.  Chem.  Soc.,  40,  264  (1918). 


254  APPLIED  COLLOID  CHEMISTRY 

hydrochloric  acid  and  acetic  acid  respectively  until  the  salt  con- 
centration is  normal,  the  swelling  is  about  the  same  in  the  two 
cases;  but  the  decrease  in  swelling  is  very  much  greater  with  the 
hydrochloric  acid  than  with  the  acetic  acid.  Until  some  of  these 
factors  have  been  taken  into  account  more  thoroughly  than  has 
yet  been  done,  it  will  be  impossible  to  discuss  the  swelling  of 
gelatine  with  any  hope  of  success.  It  must  be  remembered  that 
gelatine  may  cause  hydrolysis  of  a  salt  with  simultaneous  but 
separate  adsorption  of  base  and  acid.  This  certainly  takes  place 
with  chromic  sulphate  solutions  and  may  occur  in  other  cases. 

One  must  also  keep  in  mind  the  possibility  of  metathetical 
reactions.  Bechhold1  cites  an  experiment  in  which  0.78  grams 
gelatine  swelled  in  100  cc  of  0.05  HC1  until  it  weighed  14.61 
grams.  The  weight  was  only  about  7  grams  when  the  solution 
was  also  half-molecular  with  respect  to  potassium  chloride  and 
2.84  grams  when  half -molecular  with  respect  to  potassium  citrate. 
On  the  face  of  it,  the  potassium  citrate  has  cut  down  the  swelling 
a  great  deal  more  than  the  potassium  chloride ;  but  it  is  hardly  fair 
to  attribute  this  to  the  citrate  ion  exclusively,  because  the  solution 
really  had  a  much  lower  content  of  hydrogen  ion  than  before  and 
much  of  the  decrease  in  swelling  may  have  been  due  to  that  fact. 
The  experiments  of  Fischer  and  Hooker2  do  not  bear  on  this, 
because  they  kept  the  concentration  of  phosphate  constant  and 
varied  the  acidity  and  alkalinity.  They  found  the  greatest 
swelling  at  the  acid  and  alkali  ends  with  consequently  a  minimum 
at  some  intermediate  point. 

Jacques  Loeb3  considers  that  gelatine  forms  salts  such  as 
gelatine  chloride  or  hydrochloride  and  sodium  gelatinate,  which 
he  assumes  forms  true  solutions.  There  seems  to  be  no  possi- 
bility at  present  of  getting  people  to  agree  on  a  definition  of  true 
solution.  It  is  therefore  going  to  be  necessary  for  somebody  with 
an  adequate  knowledge  of  colloid  chemistry  to  study  the  behavior 
of  gelatine  in  detail  and  to  see  whether  a  better  explanation 
can  be  given  from  that  point  of  view.  One  set  of  experiments 
that  Loeb  made  on  the  swelling  of  gelatine  is  very  interesting.4 

1  Colloids  in  Pathology  and  Medicine,  69  (1919). 

2  Jour.  Am.  Chem.  Soc.,  40,  272  (1918). 

3  Jour.  Gen.  Physiology,  1,  39  (1918). 

4  LOEB:  Jour.  Biol.  Chem.,  31,  343  (1917). 


GELATINOUS  PRECIPITATES  AND  JELLIES  255 

Cooper's  commercial  gelatine,  when  powdered,  swells  to  a  certain 
extent  in  water  and  to  a  slightly  greater  amount  in  M/8  sodium 
chloride.  If  the  gelatine  powder  is  treated  for  a  short  time  with 
the  salt  solution  and  then  with  water  it  swells  more  than  twice 
as  much  as  in  the  salt  solution  or  in  water  alone.  Similar  results 
were  obtained  with  purified  pig's  bladder.  Loeb  considers  that 
the  first  effect  of  the  salt  solution  is  to  cause  a  chemical  reaction 
between  the  colloid  and  the  salt  which  would  give  rise  to  a  great 
swelling  if  it  were  not  checked  by  the  salt  solution.  Replacing 
the  salt  solution  by  water  makes  possible  the  swelling  due  to  the 
unspecified  reaction  between  colloid  and  salt.  This  in  itself  does 
not  mean  anything.  If  only  the  colloid  is  changed,  there  is  no 
chemical  reaction  between  the  colloid  and  the  salt.  If  there  is  a 
chemical  reaction,  then  all  the  salt  will  not  be  washed  out  and 
one  would  like  to  know  what  and  how  much  was  left  in  the  gela- 
tine. It  seems  probable  that  there  was  some  hydrolysis,  and 
that  washing  out  the  sodium  chloride  left  the  gelatine  slightly 
more  alkaline  than  before,  in  which  case  an  increased  swelling  is 
what  ought  to  take  place.  This  hypothesis  receives  some  con- 
firmation from  the  fact  that  a  preliminary  treatment  of  pig's 
bladder  with  M/16  disodium  tartrate  caused  nearly  twice  as 
much  swelling  as  the  preliminary  treatment  with  M/8  sodium 
chloride.  Since  tartrate  ions  are  presumably  adsorbed  much  less 
by  gelatine  than  chlorine  ions,  one  would  expect  the  gelatine  to 
retain  more  alkali  when  treated  with  sodium  tartrate  than  when 
treated  with  sodium  chloride.  The  in  reased  swelling  is  there- 
fore what  might  be  expected.  Before  one  can  accept  this  expla- 
nation it  is  necessary  to  show  experimentally  that  alkali  is 
adsorbed  in  both  cases  and  that  more  alkali  is  taken  up  from  the 
tartrate  solution  than  from  the  chloride  solution.  It  has  also 
been  suggested  that  the  preliminary  treatment  with  the  salt 
solution  washed  out  some  of  the  lime  salts  in  the  gelatine. 

Loeb1  has  done  a  number  of  brilliant  experiments  which  bring 
out  clearly  the  importance  of  considering  the  hydrogen  ion 
concentration  in  all  work  with  gelatine.  He  finds  that  the 
isoelectric  point  for  gelatine  lies  at  a  hydrogen  ion  concentration 
of  2  X  10~7,  or  pH  —  4.7  in  Sorensen's  logarithmic  notation. 
Under  the  conditions  of  the  experiments  Loeb  found  that  on  the 

1  Jour.  Gen.  Physiol.  1,  39,  237,  353,  483,  559  (1918-19). 


256  APPLIED  COLLOID  CHEMISTRY 

acid  side  of  the  isoelectric  point  only  anions  of  neutral  salts  are 
taken  up  and  on  the  alkaline  side  of  the  isoelectric  point  only 
cations.  Since  the  Hofmeister  series  calls  for  an  effect  due  to 
both  ions  of  a  neutral  salt  on  the  swelling  of  gelatine,  Loeb 
concludes  that  the  Hofmeister  series  is  a  delusion  and  a  snare. 
This  does  not  follow  at  all.  Loeb  is  working  at  such  extreme 
dilutions  that  the  specific  effects  of  all  ions  but  hydrogen  and 
hydroxyl  ions  are  practically  negligible.  In  acid  solutions 
only  anions  are  taken  up  and  in  alkaline  solutions  only  cations. 
Loeb  recognizes  the  specific  effect  of  iodine  ions  over  chlorine 
ions  in  causing  the  liquefaction  of  gelatine;  but  he  considers  that 
liquefaction  stands  in  no  necessary  relation  to  swelling,  an 
assumption  which  will  be  shared  by  few.  With  higher  salt 
concentrations  Loeb  will  undoubtedly  get  entirely  different  re- 
sults. His  conclusions  as  to  the  existence  of  definite  compounds 
depend  on  the  assumption  that  he  is  dealing  with  true  solutions 
and  will  fall  with  that  assumption.  While  it  is  not  possible 
to  accept  his  theoretical  conclusions,  his  experiments  with 
silver  nitrate  and  with  potassium  ferrocyanide,  to  take  two 
striking  cases,  are  marvellous  as  an  example  of  technique.  The 
so-called  osmotic  pressure  measurements  are  apparently  meas- 
urements of  swelling  and  not  of  osmotic  pressure. 

Since  the  swelling  and  shrinking  are  practically  reversible,  the 
gelatine  jellies  do  not  become  opaque  at  some  point  as  was  the 
case  with  the  silica  gels.  Katz1  has  made  some  experiments  on 
the  vapor  pressures  of  gelatine  solutions.  He  obtained  a  smooth 
pressure-concentration  curve  with  a  point  of  inflection.  With  an 
83  percent  jelly  the  partial  pressure  of  the  water  vapor  was 
about  half  that  of  saturated  water  vapor  and  for  a  90  percent 
jelly  the  partial  pressure  was  about  one-tenth  that  of  saturated 
water  vapor.  The  behavior  of  a  gelatine  j  elly  in  liquid  water  and  in 
saturated  vapor  has  already  been  discussed  elsewhere.2  If  the 
gelatine  jelly  is  hardened  somewhat  by  replacing  the  water  with 
benzene,3  it  behaves  in  many  respects  like  the  silicic  acid  jellies. 

The  pressures  that  can  be  exerted  by  a  swelling  colloid  may  be 
very  large.  We  know  how  a  growing  vine  will  displace  stones.  In 

1  Zeit.  Elektrochemie,  18,  800  (1911). 

2  See  page  75. 

SBACHMANN:  Zeit.  Kolloidchemie,  23,  85  (1918). 


GELATINOUS  PRECIPITATES  AND  JELLIES 


257 


some  experiments  on  dried  seaweeds,  it  was  found  that  water 
was  taken  up  against  a  pressure  of  forty  atmospheres  and  Rode- 
wald  states  that  starch  will  swell  against  a  pressure  of  2500 
atmospheres.  When  gelatine  dries  in  contact  with  glass,  it 
may  pull  pieces  of  glass  off  the  surface.  Posnjak1  has  made  some 
experiments  on  the  amount  of  water  with  which  gelatine  is  in 
equilibrium  under  different  external  pressures  and  on  the  cor- 
responding behavior  of  raw  Para  rubber  in  different  organic  liquids. 
The  data  are  given  in  Tables  XLVIII-XLIX,  the  pressures  being 
centimeters  of  mercury  and  the  concentrations  being  grams  of 

TABLE  XLVIII. — SWELLING  OF  GELATINE  IN  WATER 

P  =  external  pressure  in  cm  Hg. 
C  =  grams  water  per  gram  gelatine. 


P       

38.3 

82  4 

156 

240 

303 

377 

C  

2.56 

2.06 

1.46 

1.28 

1.10 

0.92 

TABLE  XLIX. — SWELLING  OF  RUBBER  IN  ORGANIC  LIQUIDS 

P  =  external  pressure  in  cm.  Hg. 
C  =  grams  liquid  per  gram  rubber. 


Liquid 

P  =  82.4 
LC 

P  =  156 
C 

P  =  240 
C 

P  =  377 
C 

Ether 

2  40 

1  79 

1  49 

Benzene  

4.41 

3.28 

2.72 

2.09 

Thiophene  

5.32 

4  00 

3.32 

2.71 

Acetylene  dichloride.  .  . 
Chloroform  

7.35 
9.31 

5.22 
7.07 

3.42 

4.58 

Carbon  tetra  chloride  .  . 

11.06 

8.47 

6.69 

liquid  per  gram  of  gelatine  or  rubber.  As  was  to  be  expected,  the 
liquid  content  decreases  with  increasing  pressure.  It  would  not 
be  safe  to  extrapolate  from  these  data  to  the  pressures  neces- 
sary to  squeeze  out  the  last  drops  of  liquid;  but  they  would  evi- 
dently be  very  high.  The  specific  effects  of  the  different  liquids 
are  quite  marked  in  the  case  of  rubber,  nearly  five  times  as 
much  carbon  tetrachloride  being  taken  up  as  ether.  Owing  to 
the  differences  in  the  densities  of  the  two  liquids,  this  is  of  course 

i  Kolloidchemische  Beihefte,  3,  417  (1912). 
17 


258  APPLIED  COLLOID  CHEMISTRY 

not  a  measure  of  the  amount  of  swelling.  With  an  external  pres- 
sure of  156  cm  Hg,  Posnjak  found  that  the  amount  of  rubber  in 
grams  per  liter  was  283  with  ether  and  150  with  carbon 
tetrachloride. 

The  taking  up  of  liquid  may  be  accompanied  by  a  marked 
evolution  of  heat.  In  Table  L  are  some  figures  by  Rodewald 
for  the  heat  evolved  when  starch  swells  in  water. 

TABLE  L. — HEAT  EVOLUTION  WITH  STARCH  AND  WATER 


%H2O 

Calories  per  gram  starch 

0.23 

28.11 

3.23 

20.97 

8.16 

12.43 

12.97 

7.37 

19.52 

2.91 

We  do  not  know  at  all  what  the  solubility  is  of  gelatine  in  water 
or  of  water  in  gelatine,  except  that  it  must  be  small.  Experi- 
ments have  been  made  to  determine  these  values  by  pressing  the 
gelatine  and  analyzing  the  supernatant  liquid  when  a  jelly  con- 
tracts; but  these  data  mean  nothing  because  there  is  no  proof 
that  the  so-called  solution  does  not  contain  the  substance  in  a 
colloidal  form. 

DIFFUSION 

In  dilute  gelatine  solutions  the  rate  of  diffusion  of  electrolytes 
is  very  nearly  the  same  as  in  pure  water;  but  this  does  not  hold 
as  the  jelly  is  made  more  concentrated.  The  electrical  conduct- 
ance is  also  less  in  gelatine  solutions  than  in  water.1 

Although  diffusion  takes  place  readily  in  gelatine  jellies,  some 
curious  phenomena  have  been  observed  under  special  conditions. 
Liesegang2  coated  a  glass  plate  with  gelatine  containing  a  small 
amount  of  potassium  or  ammonium  bichromate.  When  a  drop 
of  concentrated  silver  nitrate  solution  is  placed  on  the  gelatine, 
the  silver  nitrate  diffuses  into  the  gelatine  and  reacts  with  the 
bichromate.3  Instead  of  forming  a  continuous  precipitate,  the 
silver  chromate  comes  down  in  concentric  rings  which  are  called 

1  DUMANSKI:  Zeit.  phys.  Chem.,  60,  553  (1907). 

2  Chemische  Reaktionen  in  Gallerten,  1898. 

3  BECHHOLD:  Colloids  in  Biology  and  Medicine,  261  (1919). 


GELATINOUS  PRECIPITATES  AND  JELLIES  259 

Liesegang's  rings,  or  rhythmic  banding.  A  similar  experiment 
can  be  carried  out  in  a  test-tube  and  it  is  then  possible  to  get  as 
many  as  twenty  or  more  parallel  membranes.  Ostwald1  at- 
tempted to  account  for  the  phenomenon  by  supposing  that  the 
silver  chromate  remained  in  supersaturated  solution,  diffusing 
along  with  the  silver  nitrate  until  the  metastabile  limit  is  reached, 
when  it  precipitates.  This  ingenious  hypothesis  has  not  proved 
satisfactory,  and  there  is  as  yet  no  entirely  satisfactory  explana- 
tion of  the  phenomenon.  Microscopic  examination  shows  that 
the  apparently  clear  spaces  between  the  concentric  rings  are  not 
absolutely  free  from  crystals  of  the  insoluble  compound,  but 
contain  very  fine  crystals.  The  phenomenon  varies  with  the 
salt  used,  sodium  chloride  not  giving  rings,  and  with  the  nature 
of  the  jelly.  The  reaction  between  silver  nitrate  and  potassium 
bichromate  gives  rings  or  layers  in  gelatine  but  not  in  agar,  while 
lead  nitrate  and  potassium  chromate  give  layers  in  agar  and  not 
in  gelatine.  Neither  of  these  reactions  gives  rise  to  layers  in 
silicic  acid,2  though  other  reactions  do.  Bradford3  believes  that 
one  of  the  reacting  solutes  is  adsorbed  by  the  layer  of  precipitate, 
thus  giving  a  zone  practically  free  from  it.  There  is  of  course 
no  objection  to  the  assumption;  but,  as  Hatschek  points  out, 
this  explanation  and  that  of  Ostwald  leave  the  specific  effect  of 
the  jelly  entirely  out  of  account. 

Rhythmic  banding  is  important,  because  it  is  probable  that 
something  of  this  sort  is  the  cause  of  the  banded  structure  ob- 
served in  agates  and  elsewhere.  It  is  worth  noting  that  Bech- 
hold  claims  that  the  phenomenon  does  not  necessarily  have 
anything  to  do  with  diffusion  because  similar  structures  can  be 
produced  in  jellies  by  crystallization  of  sodium  phosphate  or  by 
freezing  water.  Holmes4  has  made  a  large  number  of  crystalliza- 
tions in  silicic  acid  jelly.  He  has  also  put  forward  a  theory  of 
rhythmic  banding,5  which  unfortunately  does  not  account  for 
the  fact,  recorded  in  the  same  paper,  that  colloidal  gold  gives 
three  colored  bands,  red,  purple  and  blue,  before  repeating. 

1  Lehrbuch  allgem.  Chemie,  2d  Ed.,  2,  778. 

2  HATSCHEK:  Second  Report  on  Colloid  Chemistry,  21  (1919). 

3  Biochem.  Jour.,  10,  169  (1916);  11,  14  (1917). 
*  Jour.  Phys.  Chem.,  21,  709  (1917). 

5  HOLMES:  Jour.  Am.  Chem.  Soc.,  40,  1187  (1918). 


CHAPTER  IX 
EMULSIONS  AND  FOAMS 

EMULSIONS 

When  considering  the  adsorption  of  solids  by  liquids,  it  was 
pointed  out  that  certain  solids  will  pass  into  the  interface  between 
two  liquids  and  will  be  held  there  quite  firmly.  This  will  also 
happen  if  the  solid  is  present  in  so  finely  divided  a  state  that  it 
forms  a  colloidal  solution  in  one  or  both  of  the  liquid  phases,  as 
was  found  experimentally  by  Winkelblech1  who,  however,  was 
very  much  puzzled  by  the  phenomenon.  He  shook  a  dilute 
solution  of  gela-tine  with  benzene  and  found  a  film  of  gelatine  at 
the  dineric  interface.  The  method  was  so  sensitive  that  0.06  mg 
gelatine  could  be  detected  when  working  with  10  cc  lots.  Win- 
kelblech proposed  to  use  this  as  a  method  of  analysis;  but  this  is 
not  satisfactory  because  the  sensitiveness  varies  with  changes  in 
the  gelatine  or  in  the  acidity  of  the  solution.  The  more  readily 
the  substance  is  peptized  by  one  of  the  liquids,  the  less  readily 
it  will  go  into  the  interface.  We  have  already  seen  that  solid 
gelatine  is  not  peptized  readily  by  water.  Similar  results  were 
obtained  with  albumin,  water-soluble  starch,  soap,  colophonium, 
the  proteins  in  beer,  and  with  tannic  acid,  though  not  with  gallic 
acid.  As  precipitating  liquids  could  be  used  petroleum,  liquid 
paraffin,  benzene,  carbon  tetrachloride,  and  chloroform,  but  not 
ether.  Winkelblech  did  not  find  out  why  ether  did  not  work; 
but  it  is  interesting  to  note  that  Miller  and  McPherson2  found 
that  arsenic  sulphide  forms  a  colloidal  solution  both  in  ether 
and  in  water,  and  that  on  shaking  a  colloidal  solution  of  arsenic 
sulphide  in  water  with  ether,  the  colloid  distributes  itself  through 
the  two  phases.  Winkelblech  also  succeeded  in  the  partial  extrac- 
tion of  colloids  from  organic  liquids  by  shaking  colloidal  solutions 
of  fat,,  with  water,  the  fats  forming  a  film  at  the  interface. 

1  Zeit.  angew.  Chem.,  18,  1953  (1906). 

2  Jour.  Phys.  Chem.,  12,  706  (1908). 

260 


EMULSIONS  AND  FOAMS  261 

Biltz  and  Krohnke1  made  some  experiments  to  see  whether 
the  colloidal  organic  matter  in  sewage  could  not  be  extracted  by 
shaking  with  organic  liquids  but  they  found  that  only  about  30 
percent  went  into  the  interface,  so  the  method  never  came  into 
technical  use.2 

In  these  experiments  of  Winkelblech  the  amount  of  organic 
liquid  is  large  and  the  drops  formed  on  shaking  are  not  very 
small.  The  tendency  is  to  form  a  coarse  emulsion  but  this  is  not 
stable  and  cracks,  the  colloid  then  passing  into  the  interface. 
An  emulsion  consists  of  drops  of  one  liquid  suspended  in  another 
liquid.  When  these  drops  are  stabilized  by  an  electrical  charge 
they  behave  like  any  other  colloidal  solution  of  the  same  type.3 
In  most  cases  there  is  an  actual  film  around  the  globules  and  this 
keeps  them  from  coalescing.  It  is  evident  that  with  any  pair  of 
non-miscible  liquids  there  may  be  two  sets  of  emulsions,  one  with 
drops  of  oil  suspended  in  water  and  one  with  drops  of  water 
suspended  in  oil,  the  word  oil  being  used  here  to  mean  the  other 
liquid.  The  water-in-oil  emulsion  may  be  important  in  certain 
cases.  Wa.  Ostwald4  has  pointed  out  that  if  one  treats  wood  with 
creosote  emulsified  in  water,  the  water  comes  in  direct  contact 
with  the  wood,  which  is  not  what  one  wants.  If  one  takes  an 
emulsion  of  water  in  creosote,  the  creosote  comes  in  direct  contact 
with  the  fibers  of  the  wood,  which  is  much  more  satisfactory.  It 
seems  probable  that  in  the  case  of  a  kerosene  spray,  one  type  is 
better  than  the  other,  though  that  point  seems  not  to  have  been 
considered.  The  necessary  conditions  for  forming  a  stable  emul- 
sion are  that  the  drops  shall  be  so  small  that  they  will  stay  sus- 
pended and  that  there  shall  be  a  sufficiently  viscous  film  round 
each  drop  to  keep  the  drops  from  coalescing.  An  emulsifying 
agent  is  therefore  a  substance  which  goes  into  the  interface  and 
produces  a  film  having  satisfactory  physical  properties.  If  the 
adsorption  of  the  emulsifying  agent  lowers  the  surface  tension 
on  the  water  side  of  the  interface  more  than  it  does  on  the  oil 
side,  the  interface  will  tend  to  curve  so  as  to  be  convex  on  the 

1  Zeit.  angew.  Chem.,  20,  883  (1907). 

2Cf.  BRIGGS:  Jour.  Phys.  Chem.,  19,  210  (1915). 

3  ELLIS:  Zeit.  phys.  Chem.,  80,  597  (1912);  Powis:  Ibid.,  89,  9\  179 
(1914). 

4  Zeit.  Kolloidchemie,  6,  103  (1910). 


262  APPLIED  COLLOID  CHEMISTRY 

water  side,  and  we  shall  have  a  tendency  to  emulsify  oil  in 
water.  If  the  adsorption  of  the  emulsifying  agent  lowers  the 
surface  tension  on  the  oil  side  of  the  interface  more  than  it 
does  on  the  water  side,  the  interface  will  tend  to  curve  so  as 
to  be  concave  on  the  water  side,  and  we  shall  have  a  tendency 
to  emulsify  water  in  oil.  The  simplest  way  then  to  emulsify 
oil  in  water  is  to  add  a  water-soluble  colloid  which  is  adsorbed 
strongly  at  the  interface  and  the  simplest  way  to  emulsify  water 
in  oil  is  to  add  an  oil-soluble  colloid  which  is  adsorbed  strongly 
in  the  interface.  As  a  matter  of  fact  this  is  the  way  in  which 
almost  all  emulsions  are  made.  Since  most  emulsifying  agents  will 
not  peptize  the  liquid  by  themselves,  it  is  usually  necessary  to  break 
up  the  liquid  into  fine  drops  by  some  form  of  mechanical  agitation. 

Sodium  and  potassium  oleates  are  water-soluble  colloids  and 
they  are  excellent  for  emulsifying  oils  in  water.  The  gums  are 
also  water-soluble  colloids  and  certain  of  them  are  much  used  in 
pharmaceutical  work  for  emulsifying  oils  in  water.  Though 
gelatine  is  a  water-soluble  colloid,  it  is  not  used  ordinarily  as  an 
emulsifying  agent  probably  because  there  is  too  much  danger  of 
putrefaction.  By  adding  salt  to  a  soap  solution,  the  soap  film 
can  be  made  to  come  out  so  brittle  that  it  will  not  form  a 
satisfactory  emulsion. 

Pickering1  made  emulsions  of  kerosene  in  water  with  small 
amounts  of  soap  as  the  emulsifying  agent.  With  70-80  percent 
of  kerosene,  the  mass  was  viscous  and  with  99  percent  oil  it  was 
as  stiff  as  blanc-mange,  and  could  be  cut  into  cubes  which  would 
stand  alone.  When  a  lump  of  this  solidified  kerosene  was  left 
standing  in  dry  air,  it  liquefied  because  the  water  evaporated 
from  the  soap  film  which  then  cracked,  letting  the  kerosene  run 
out.  The  solidified  alcohol  which  is  so  much  used  nowadays  is 
not  an  emulsion,  though  it  does  contain  some  water.  It  is  es- 
sentially an  alcoholic  soap  jelly  and  presumably  does  not  con- 
tain alcohol  dispersed  as  drops. 

Calcium  and  magnesium  oleates  form  colloidal  solutions  in  oil 
and  can  therefore  be  used  to  emulsify  water  in  oil.  Rosin  and 
the  resinates  act  in  the  same  way  and  the  water  in  ready-mixed 
paints  is  an  emulsion  of  water  as  drops  in  linseed  oil  containing 
rosin.  It  is  a  simple  matter  to  add  fifty  percent  of  water  to  a 

1  Jour.  Chem.  Soc.,  91,  2002  (1902). 


EMULSIONS  AND  FOAMS  263 

ready-mixed  white  lead  without  changing  the  look  of  the  mixture 
much,  because  the  oil  is  the  external  phase.  If  water  is  stirred 
into  a  heavy  lubricating  oil  containing  a  lime  soap,  the  water 
will  be  emulsified  in  the  oil  and  will  change  it  to  a  grease,  instead 
of  decreasing  the  viscosity  as  might  perhaps  have  been  expected. 
Lanolin,  or  purified  wool  fat,  will  emulsify  water  and  can  easily 
be  made  to  take  up  eighty  percent  of  it.  Nobody  has  deter- 
mined what  the  emulsifying  agent  is  in  this  case.  Friedrich1  pro- 
poses making  printing  inks  with  about  half  the  normal  content  of 
linseed  oil,  turpentine,  drying  oils,  etc.  by  diluting  with  water 
emulsified  in  wool  fat  and  adding  a  little  more  coloring  matter. 
The  degras  which  is  used  in  dubbing  leather  is  apparently  an 
emulsion  of  water  in  oil  because  Fahrion2  states  that  normal 
degras  should  contain  15-20  percent  water  but  that  English  sod- 
oil  may  run  30-40  percent  water  and  the  white  degras  from  the 
German  chamois  leather  works  may  go  as  high  as  40-50  percent 
water. 

The  water  in  butter  may  be  emulsified  by  means  of  a  calcium 
soap;  but  more  probably  the  water  is  merely  dispersed  as  drops  in 
a  viscous  medium  without  any  true  emulsion  being  formed.  Milk 
is  an  emulsion  with  butter  fat  as  drops  in  a  water  phase.  It  is 
possible  to  make  imitation  fresh  milk  by  emulsifying  renovated 
butter  fat  in  skimmed  milk.  It  is  claimed  that  some  day  the 
milk  problem  of  the  large  cities  will  be  solved  by  shipping  butter 
and  a  suitable  milk  powder  to  the  cities  and  adding  the  water 
there,  since  there  is  a  very  large  difference  in  cost  between  sending 
milk  powder  by  slow 'freight  and  water  by  what  is  practically 
express,  to  say  nothing  of  the  difference  in  keeping  qualities. 
Mayonnaise  is  an  emulsion  of  oil  in  water  with  egg  as  the  emulsi- 
fying agent.  Cod-liver  oil  is  of  course  an  emulsion  of  oil  in  water. 

It  is  not  always  easy  to  tell  by  inspection,  whether  water  is 
external  or  internal  phase  in  a  given  emulsion.  A  good  way, 
devised  by  Briggs,  is  to  examine  the  emulsion  under  the  micro- 
scope,3 stirring  in  gently  a  little  water  or  a  little  oil.  The  one 
that  is  external  phase  will  mix  readily  with  the  emulsion,  while 
the  other  will  not. 

1  Jour.  Soc.  Chem.  Ind.,  36,  296  (1917). 

2  Ibid.,  30,501  (1911). 

3  NEWMAN:  Jour.  Phys.  Chem.,  18,  34  (1914). 


264  APPLIED  COLLOID  CHEMISTRY 

The  emulsion  jellies  are  of  course  all  of  the  honeycomb  type 
and  it  is  a  pity  that  none  of  them  have  yet  been  examined  iii  the 
ultramicroscope.  If  they  had  been  shown  to  be  quite  different 
in  appearance  from  the  gelatine  and  the  silica  jellies,  for  instance, 
this  would  have  been  a  strong  argument  in  favor  of  the  view  that 
the  latter  jellies  are  of  a  different  type.  If  no  marked  difference 
could  be  discovered,  as  would  undoubtedly  be  the  case,1  it 
would  have  been  recognized  long  ago  that  the  evidence  of  the 
ultramicroscope  is  worthless  in  matters  of  this  sort. 

In  making  emulsions  of  oil  in  water  by  means  of  sodium  oleate, 
it  must  be  remembered  that  the  soap  does  not  go  entirely  into 
the  interface.  There  is  always  a  relation  between  the  concen- 
tration of  the  soap  in  the  interface  and  the  amount  still  peptized 
in  the  water.2  It  is  a  case  of  the  distribution  of  soap  between 
the  water  and  the  interface.  Anything  that  cuts  down  the  peptiz- 
ing  action  of  the  water  on  the  soap  will  increase  the  amount  going 
into  the  interface  and  anything  increasing  the  peptizing  action 
of  the  water  will  decrease  the  amount  of  soap  in  the  interface. 
It  is  not  a  question  of  osmotic  pressure  or  of  solubility. 

While  most  emulsions  are  made  with  gelatinous  colloids  as 
emulsifying  agents,  this  is  not  necessary  theoretically.  Any- 
thing that  will  go  into  the  interface  and  make  it  sufficiently  viscous 
will  give  the  same  result.  If  we  can  put  enough  of  a  fine  powder 
into  the  interface,  we  shall  have  a  plastic  mass  there,  which  will 
stabilize  the  emulsion.  This  has  been  done  by  Pickering3  who 
obtained  emulsions  of  oil  in  water  using  powdered,  basic  copper 
or  iron  sulphate  as  the  emulsifying  agents.  The  emulsions  were 
not  very  good  ones  but  they  were  emulsions.  Sheppard4  has 
obtained  similar  results,  emulsifying  nitrobenzene  in  aqueous 
sulphuric  acid  by  means  of  lead  sulphate.  When  mayonnaise  is 
made  with  the  powdered  yolk  of  a  hard-boiled  egg,  the  resulting 
emulsion  comes  under  this  head.  Schlaepfer5  and  Moore6  have 
emulsified  water  in  kerosene  by  means  of  carbon  black. 

1  BACHMANN:  Zeit.  Kolloidchemie,  23,  89  (1918). 

2  BBIGGS:  Jour.  Phys.  Chem.,  19,  210  (1915). 

3  Jour.  Chem.  Soc.,  91,  2010  (1907);  Zeit.  Kolloidchemie,  7,  14  (1910). 
*  Jour.  Phys.  Chem.,  23,  634  (1919). 

5  Jour.  Chem.  Soc.,  113,  522  (1918). 

6  Jour.  Am.  Chem.  Soc.,  41,  940  (1919). 


EMULSIONS  AND  FOAMS  265 

The  pharmacists  make  great  use  of  emulsions  of  oil  in  water 
and  the  books  on  pharmacy  are  filled  with  elaborate  details  as  to 
the  method  of  making  emulsions.  There  are  two  general  me- 
thods in  vogue  and  both  give  good  results,  although  they  appear 
mutually  contradictory.  In  the  American  method  the  gum, 
which  is  used  as  an  emulsifying  agent,  is  peptized  and  the  resulting 
colloidal  solution  is  placed  in  a  large  mortar.  The  oil  is  added  a 
little  at  a  time  or  in  four  separate  portions  with  continuous  stir- 
ring. If  one  starts  stirring  to  the  right,  one  must  continue  stirring 
to  the  right  or  no  emulsion  will  be  formed.  Some  books  go  so  far 
as  to  say  that  a  left-handed  man  cannot  make  an  emulsion,  but 
that  seems  a  bit  absurd.  In  the  Continental  method,  the  gum 
is  ground  in  a  mortar  with  the  oil,  in  which  it  does  not  dissolve, 
and  enough  water  to  emulsify  all  the  gum  is  then  added  at  once. 

Since  the  essential  condition  for  making  an  emulsion  is  to 
break  the  oil  into  sufficiently  fine  drops  and  coat  them  with  the 
gum  solution,  there  is  no  necessary  reason  for  adding  the  oil  in 
separate  portions  in  the  American  method.  When  the  constitu- 
ents are  mixed  in  a  bottle  and  shaken  in  a  shaking  machine,  it 
is  easy  to  make  an  emulsion  if  the  amount  of  oil  is  not  too  large. 
With  a  mechanical  shaker1  giving  400  shakes  per  minute  and  a 
one  percent  solution  of  sodium  oleate,  80  percent  by  volume  of 
benzene  could  be  emulsified  in  fifteen  minutes  but  it  took  two 
hours  to  emulsify  96  percent  of  benzene,  and  99  percent  of 
benzene  was  not  completely  emulsified  after  eight  hours  of 
continuous  shaking.  If  the  shaking  is  done  intermittently  by 
hand  so  that  the  unemulsified  benzene  can  separate  at  intervals, 
the  number  of  shakes  can  be  cut  down  enormously,  from  48,000 
to  80  in  the  case  of  the  96  percent  emulsion  and  from  6000  to  7 
in  the  case  of  the  80  percent  one.  Continuous  shaking  in  a 
bottle  disintegrates  both  benzene  and  soap  solution  into  drops, 
the  latter  of  which  is  disadvantageous.  Since  the  soap  solution 
is  the  continuous  phase  in  these  emulsions,  any  treatment  which 
breaks  up  the  soap  solution  retards  emulsification.  When  ben- 
zene is  dispersed  into  drops  in  the  soap  solution,  the  soap  coats 
the  drops  and  prevents  them  from  coalescing,  while  there  is 
nothing  to  prevent  the  rapid  coalescing  of  the  water  drops  in  the 
benzene.  The  first  few  shakes  cause  the  most  disintegration  of 

1  BRIGGS:  Jour.  Phys.  Chem.,  24,  120  (1920). 


26G  APPLIED  COLLOID  CHEMISTRY 

the  benzene  and  the  least  disintegration  of  the  soap  solution ;  con- 
sequently intermittent  shaking  gives  the  best  results.  The 
pharmacist  was  doing  intermittent  stirring  unconsciously  when 
he  added  the  oil  in  separate  portions.  The  reason  for  using  a 
mortar  that  is  apparently  much  too  large  is  to  get  a  large  surface 
and  thus  to  facilitate  emulsification.  A  similar  result  can  be 
obtained  by  putting  the  mixture  in  a  cylindrical  glass  bottle, 
with  or  without  glass  beads,  and  rolling  the  bottle. 

These  experiments  throw  some  light  on  the  making  of  mayon- 
naise. Since  mayonnaise  is  essentially  an  emulsion  of  oil  in 
water  (vinegar)  with  egg  as  the  emulsifying  agent,  it  ought  to 
behave  like  any  other  emulsion  and  so  it  does  for  the  experts  in 
the  Departments  of  Home  Economics.  They  can  add  the  in- 
gredients in  any  order,  all  at  once  or  in  separate  portions,  hot  or 
cold,  and  the  mayonnaise  always  comes.  On  the  other  hand, 
these  same  experts  do  not  train  their  pupils  so  that  these  latter 
can  make  mayonnaise  every  time.  It  seems  certain  that  the 
experts  do  something  or  other  unconsciously  which  they  conse- 
quently do  not  tell  to  their  pupils.  Probably  the  expert  is  so 
sure  of  the  result  that  she  works  leisurely  without  being  hurried 
or  flurried,  and  is  practically  doing  intermittent  stirring.  The 
person  who  is  not  an  expert  and  who  is  uncertain  of  the  outcome 
probably  goes  at  her  task  so  vigorously  as  to  defeat  her  object  in 
many  cases.  While  this  explanation  has  not  been  tested,  one 
expert  said  that  she  had  found  that  if  the  materials  were  beaten 
well  together  and  then  allowed  to  stand  for  a  moment  or  two,  a 
couple  of  swishes  would  make  the  mayonnaise.  I  have  been 
told  that  Bearnaise  sauce  is  an  emulsion  of  melted  butter  in 
tarragon  vinegar  without  any  egg  and  that  it  is  very  easy  to  make. 
If  this  is  so,  tarragon  vinegar  must  contain  a  good  deal  more  of 
some  emulsifying  material,  probably  a  tannin,  than  ordinary 
vinegar. 

The  Continental  method  of  making  emulsions  has  also  been 
studied  by  Briggs1  who  finds  that  the  object  of  grinding  the  oil 
with  the  solid  gum  is  merely  to  increase  the  interface  between 
oil  and  water  by  suspending  in  the  oil  finely  divided  solids  which 
are  wetted  preferentially  by  water.  The  gum  acts  both  as 
finely  divided  solid  and  as  emulsifying  agent.  It  is  possible  to 

1  Jour.  Phys.  Chem.,  24,  147  (1920). 


EMULSIONS  AND  FOAMS  267 

get  the  same  results  by  grinding  the  oil  with  sand  and  adding  a 
colloidal  solution  of  gum,  but  the  resulting  emulsion  then  contains 
the  sand. 

Since  sodium  oleate  emulsifies  oil  in  water  and  calcium  oleate 
emulsifies  water  in  oil,  a  mixture  of  the  two  oteates  will  behave 
differently,  depending  on  the  relative  amounts.  There  will  also 
be  some  ratio  of  calcium  to  sodium  at  which  the  two  oleates  will 
practically  balance  each  other  and  the  slightest  relative  change 
will  change  the  type  of  the  emulsion.  This  has  been  proved 
experimentally  by  Clowes1  who  worked  with  a  sodium  soap  and 
calcium  chloride.  This  antagonistic  action  of  sodium  and  cal- 
cium salts  with  reference  to  emulsions  throws  light  on  some  per- 
plexing physiological  problems.  Jacques  Loeb  showed  that 
certain  marine  organisms  died  in  sodium  chloride  or  calcium 
chloride  solutions  which  were  isotonic  with  sea-water;  but  flour- 
ished when  there  was  a  definite  ratio  of  sodium  to  calcium,  a 
result  which  could  not  be  explained  on  the  basis  of  osmotic 
pressure.  If  we  -consider  protoplasm  as  consisting  of 
lipoids  (oil)  and  water,  we  shall  have  an  emulsion  of  oil  in  water 
in  presence  of  sodium  salts  and  of  water  in  oil  in  presence  of 
calcium  salts.  When  the  sodium  and  calcium  salts  are  present 
in  a  definite  ratio,  there  will  be  a  balancing  between  these  two 
types  of  emulsion,  and  it  may  well  be  that  this  critical  state  is 
the  one  which  is  conducive  to  life  and  growth.  As  a  matter  of 
fact,  Clowes  found  that  the  ratio  of  sodium  and  calcium  salts 
necessary  to  produce  a  balancing  between  the  two  types  of  emul- 
sions when  working  with  oil,  water  and  soap,  was  practically  the 
same  as  that  found  in  sea-water.  This  indicates  a  very  close 
connection  between  the  two  sets  of  phenomena.  Osterhout2  at 
Harvard  has  shown  that  the  specific  electric  conductivity  of 
certain  seaweeds  is  increased  by  addition  of  sodium  salts  and 
decreased  by  the  addition  of  calcium  salts.  If  the  hypothetical 
emulsion  changed  to  one  of  oil  in  water,  the  conductivity  should 
increase,  and  it  should  decrease  if  the  emulsion  changed  to  one 
of  water  in  oil.  Clowes3  has  succeeded  in  duplicating  Oster- 
hout's  results  by  impregnating  filter  paper  with  an  emulsion 

1  Jour.  Phys.  Chem.,  29,  407  (1916). 

2  Science,  (2)  35,  112;  36,  350  (1912). 

3  Proc.  Soc.  Exp.  Biol.  Med.  15,  108  (1918). 


268  APPLIED  COLLOID  CHEMISTRY 

of  oil,  water,  and  soap,  to  which  he  afterwards  added  sodium 
and  calcium  salts. 

When  a  badly  made  emulsion  is  allowed  to  stand,  it  decom- 
poses progressively.  This  seems  to  be  due  to  the  fact  that  when 
the  free  oil  comes  in  contact  with  the  film  around  the  oil  drops, 
it  tends  to  make  it  bend  in  the  opposite  direction,  thereby  crack- 
ing the  emulsion.  Temperature  changes  probably  play  a  part  in 
causing  the  ageing  and  cracking  of  the  films;  but  there  has  been 
no  careful  study  of  this  point.  All  we  know  is  that  anything 
which  destroys  the  film  destroys  the  emulsion.  That  is  why 
acid  destroys  a  soap  emulsion.  In  the  case  of  oil  emulsions  in 
water,  we  get  cataphoresis  and  the  emulsions  can  be  broken 
down  by  the  action  of  a  high-tension  direct  current.1  In  the 
oil  industry  emulsions  of  one  kind  and  another  are  a  common 
occurrence  and  their  removal  is  quite  an  important  matter. 
Very  little  that  is  accurate  has  been  published  in  regard  to  this. 

One  interesting  case  in  the  technical  application  of  emulsions 
is  the  splitting  of  fats  by  superheated  steam  in  the  presence  of 
lime.  Caustic  soda  is  not  effective  because  it  is  necessary  to  use 
so  much;  but  one  percent  of  lime  is  sufficient  to  cause  complete 
hydrolysis.  No  explanation  of  this  has  been  tested  out;  but  it 
seems  certain  that  the  calcium  soap  emulsifies  the  water  in  the 
fat.  This  may  give  a  sufficiently  intimate  mixture  to  hydrolyze 
the  fats  or  the  water  globules  may  pass  into  steam,  breaking  down 
the  emulsion  which  then  forms  again.  Whatever  the  secondary 
details  may  be,  it  is  clear  that  the  formation  of  an  emulsion  of 
water  in  oil  is  the  essential  thing  because  the  oxides  of  magnesium 
and  of  zinc  act  in  a  similar  way. 

FOAMS 

In  an  emulsion  of  oil  in  water  we  have  drops  of  oil  coated  by  a 
more  or  less  viscous  film  containing  water,  with  water  outside. 
If  the  emulsion  is  not  sufficiently  fine,  the  emulsified  oil  will  rise 
to  the  surface  and  we  get  a  sort  of  cream,  which  consists  almost 
entirely  of  the  oil  drops  with  their  viscous  coating  and  with  little 
or  no  free  water.  If  we  consider  the  globules  of  oil  replaced  by 
air,  we  shall  have  a  mass  of  froth,  the  only  difference  that  the 

1  FANTO  and  STRITAB:  Jour,  prakt.  Chem.  (2)  81,  564  (1910). 


EMULSIONS  AND  FOAMS  269 

bubbles  would  be  much  finer  than  in  the  ordinary  froth.  The 
conditions  for  forming  froth  should  then  be  much  the  same  as  for 
forming  an  emulsion  and  that  is  the  case,  though  there  are  of 
course  some  differences. 

It  has  already  been  shown  that  colloidal  solutions  of  peptone 
form  a  film  of  solid  peptone  over  the  surface1  and  that  the  surface 
viscosity  of  a  saponin  solution  is  60  times  that  of  the  mass  of  the 
liquid.  The  formation  of  a  film  at  the  surface  depends  primarily 
on  the  adsorption  of  the  colloid  at  the  interface  and  not  on  the 
presence  of  air.  On  the  other  hand  the  adsorption  of  air  by 
the  colloid  will  cause  the  latter  to  rise  more  rapidly  and  will  also 
help  to  keep  it  there.  This  effect  might  be  so  strong  as  to  give 
rise  to  a  film  at  the  surface  in  certain  cases  where  none  would  be 
formed  otherwise.  Stark2  has  called  especial  attention  to  the 
adsorption  of  air  by  colloidal  particles.  We  know  that  gum 
arabic  adsorbs  air  strongly  and  sodium  palmitate  likewise. 
Findlay3  has  shown  that  carbon  dioxide  is  taken  up  under  some 
conditions  by  starch  solutions. 

Freundlich4  considers  that  no  pure  liquid  can  foam  and  in  this 
he  is  apparently  right.  He  then  deduces  that  a  low  vapor  pres- 
sure, a  low  surface  tension,  and  a  high  surface  viscosity  are 
essential  to  foam.  The  low  vapor  pressure  is  put  in  avowedly 
because  a  foam  with  a  volatile  liquid  like  ether  would  evaporate 
very  rapidly.  That  is  not  necessary  because  we  can  work  in 
closed  vessels.  Freundlich  has  put  in  the  low  surface  tension 
because  all  the  foaming  solutions  which  he  knew  about  had  lower 
surface  tensions  than  the  pure  liquid;  but  this  criterion  is  not 
sound  because  salt  solutions  will  foam.  Since  salts  may  increase 
the  fluidity  of  water,  a  high  surface  viscosity  is  not  essential.  All 
three  of  Freundlich's  criteria  thus  go  by  the  board,  which  shows 
the  danger  of  the  empirical  generalization.  On  the  other  hand 
Freundlich's  criteria  do  hold  in  practically  all  cases  if  one  con- 
fines oneself  to  fairly  permanent  foams,  which  was  what  he  was 
really  considering. 

To  get  a  foam  the  only  essential  is  that  there  shall  be  a  distinct 

1  See  page  189. 

2  Wied.  Ann.,  68,  117,  618  U899). 

3  Jour.  Chem.  Soc.,  101,  1459  (1912);  103,  636,  1170  (1913). 

4  Kapillarchemie,  302  (1909). 


270  APPLIED  COLLOID  CHEMISTRY 

surface  film,  in  other  words,  that  the  concentration  in  the  surface 
layer  shall  differ  perceptibly  from  that  in  the  mass  of  the  liquid. 
All  true  solutions  will  therefore  foam  if  there  is  a  marked  change 
of  surface  tension  with  concentration,  regardless  whethert  he 
surface  tension  increases  or  decreases.  All  colloidal  solutions 
will  foam  if  the  colloid  concentrates  in  the  interface  or  if  it  is 
driven  away  from  the  interface.  To  get  a  fairly  permanent  foam 
the  surface  film  must  either  be  sufficiently  viscous  in  itself  or 
must  be  stabilized  in  some  way.  This  can  be  done  by  introducing 
a  solid  powder  into  the  interface. 

Solutions  of  aqueous  alcohol,  acetic  acid,  sodium  chloride,  and 
sulphuric  acid  all  foam  when  shaken;  but  the  foam  is  instable. 
Soap  solutions  foam  when  shaken  and  the  foam  is,  or  may  be, 
quite  stable  owing  to  the  viscosity  of  the  soap  film.  With  sapo- 
nin  the  surface  film  is  even  more  stable.  If  we  add  to  aqueous 
alcohol  some  substance  like  lycopodium  powder  which  goes  into 
the  interface,  we  get  a  stabilized  foam.  We  can  do  the  same 
thing  with  aqueous  acetic  acid  by  adding  lamp-black.  The  pres- 
ence of  enough  of  a  finely  divided  solid  in  the  interface  will  make 
the  film  so  viscous  that  the  foam  will  be  quite  stable.  Grease 
will  help  stabilize  a  foam  in  some  cases  and  it  has  been  claimed 
erroneously  that  the  foaming  of  sulphuric  acid  solutions  is  due  to 
grease.1 

In  1857  Gladstone2  pointed  out  that  aqueous  solutions  of 
organic  substances  are  apt  to  froth  and  he  cited  beer  as  a  then 
familiar  instance.  He  did  not  realize  however  that  it  was  the  col- 
loidal matter  in  the  beer  which  caused  the  frothing.  It  has  been 
shown  by  Zeidler  and  Nauck3  that  removing  the  albumoses  from 
beer  destroys  the  foaming.  Gladstone  showed,  however,  that 
the  dissolved  air  was  not  essential  to  the  frothing.  If  this  were 
pumped  out  in  a  vacuum,  the  liquid  frothed  freely  when  shaken 
with  air.  Gladstone  states  that  aqueous  solutions  of  the  acetates 
of  iron,  copper,  lead,  and  other  metals  froth  readily,  especially 
the  ferric  acetate  solution.  As  we  know,  this  is  the  one  which 
hydrolyzes  most  readily  and  the  frothing  is  due  undoubtedly  to 
the  combined  effect  of  the  acetic  acid  and  the  hydrous  ferric  oxide. 

1  LANG:  Ber.  deutsch.  chem.  Ges.,  18,  1391  (1885). 

2  Phil.  Mag.  (4)  14,  314  (1857). 

3  Jour.  Soc.  Chem.  Ind.,  20,  260  (1901). 


EMULSIONS  AND  FOAMS  271 

Ramsden1  obtained  precipitation  of  solids  by  shaking  solutions 
of  albumin,  proteins,  gelatine,  cupric  acetate,  ferric  acetate, 
mastic,  etc.  This  is  essentially  the  same  in  principle  as  Winkel- 
blech's  later  experiments  with  a  second  liquid  phase.  The  colloid 
goes  into  the  surface,  forming  a  solid  film  which  is  so  brittle,  how- 
ever, that  it  does  not  give  a  good  emulsion  or  foam.  When  the 
emulsion  or  foam  cracks,  the  colloid  does  not  go  back  at  once  into 
colloidal  solution;  but  remains  as  a  precipitate.  An  interesting 
case  of  this  is  to  be  found  in  the  behavior  of  enzymes.  Many 
enzyme  solutions  can  be  rendered  inactive  merely  by  shaking.2 
A  rennet  solution  need  be  shaken  violently  in  a  test  tube  for 
only  two  minutes  to  destroy  most  of  its  power  to  coagulate  milk. 
The  inactivation  was  found  to  be  a  surface  phenomenon,  the 
inactivation  increasing  with  the  length  of  time  and  the  violence 
of  the  shaking.  The  volume  of  air,  the  concentration  of  the 
enzyme  and  the  temperature  are  all  influencing  factors.  The  en- 
zyme concentrates  in  the  foam  and  on  the  walls  of  the  vessel. 
Vanino  and  Hart  I3  shook  up  a  red  colloidal  solution  of  gold  and 
found  that  the  gold  first  turned  blue  and  then  precipitated  as 
brown  gold.  The  irreversible  formation  of  a  film  on  agitation 
with  air  is  an  important  thing  in  making  whipped  cream.  It 
seems  probable  though  not  proved  that  we  might  get  tiny  air 
bubbles  emulsified  in  a  liquid,  in  which  case  the  air  in  the  bubbles 
would  be  given  off  relatively  slowly  when  the  pressure  was 
relieved. 

If  we  have  two  colloids  present,  one  of  which  goes  into  the 
interface  more  readily  than  the  other,  the  first  may  keep  the 
second  out  more  or  less  completely.  Herty  and  Willard4  found 
that  resene  cuts  down  soap  lather  while  it  has  been  shown  that 
shaking  does  not  inactivate  a  rennet  solution  to  which  saponin 
has  been  added  because  the  latter  keeps  the  rennet  from  going 
into  the  interface. 

Under  adsorption  of  solid  by  liquid,  reference  was  made5  to 
the  Elmore  bulk  oil  process  of  ore  flotation  where  the  sulphide 

i  Proc.  Roy.  Soc.,  72,  156  (1903);  Zeit.  phys.  Chem.,  47,  336  (1904). 
*BECHHOLD:  Colloids  in  Biology  and  Medicine,  189  (1919). 

3  Ber.  deutsch.  chem.  Ges.,  37,  3623  (1904). 

4  Jour.  Ind.  Eng.  Chem.,  6,  895  (1914) 

5  See  page  89. 


272  APPLIED  COLLOID  CHEMISTRY 

ore  went  chiefly  into  the  interface  between  oil  and  water  while 
the  siliceous  gangue  settled  to  the  bottom.  This  was  an  expen- 
sive process  technically  because  it  called  for  about  one  ton  of  oil 
per  ton  of  ore.  The  Minerals  Separation  Company  holds  a 
patent  for  ore  separation  using  only  a  fraction  of  one  percent  of 
oil  per  ton  of  ore,  a  froth  being  formed  which  carries  up  the  values. 
If  no  ore  is  present,  bubbles  of  air  are  formed  coated  with  a  thin 
film  of  oil,  around  which  is  water.  The  oil  film  is  not  viscous 
enough  to  form  a  stable  froth.  If  we  introduce  particles  of 
sulphide  ore  into  the  oil-water  interface  or  into  the  oil  itself,  we 
get  a  stabilization  of  the  film  because  we  then  have  practically 
an  armor-plated  bubble.  The  resulting  froth  can  be  scraped  or 
shovelled  off.  It  is  said  that  about  sixty  million  tons  of  ore 
are  handled  by  this  process  every  year  in  America  alone.  In 
most  cases  acidification  of  the  solution  and  rise  of  temperature 
increases  the  effectiveness  of  the  separation.  A  blended  oil 
seems  often  to  give  better  results  than  a  straight  oil.  One 
oil  may  spread  better  over  the  surface  and  be  a  more  effective 
frothing  oil,  while  a  second  oil  may  have  a  greater  effect  in  carry- 
ing the  sulphide  particles  into  the  interface.  Although  a  good 
deal  of  experimenting  has  been  done  with  this  process,  the  theo- 
retical results  are  not  what  they  should  be,  apparently  because 
people  have  insisted  upon  looking  upon  the  whole  thing  as  very 
mysterious.  This  is  one  of  the  many  industries  involving  colloid 
chemistry  where  the  art  is  far  ahead  of  the  science.  In  Australia 
eucalyptus  oil  has  proved  very  successful.  In  this  country  so- 
called  pine  oil  has  been  used  a  great  deal  with  or  without  admix- 
ture of  mineral  oils.  While  sulphide  ores  are  the  ones  which 
have  been  worked  with  the  most,  the  principle  is  a  general  one, 
and  the  Calumet  and  Hecla  Mining  Company  are  reported  to  be 
using  the  process  for  removing  native  copper  from  the  tailings. 
If  the  amount  of  air  is  insufficient,  the  oil  may  cause  the  ore 
particles  to  agglomerate  and  sink.  This  was  patented  by  Catter- 
mole  but  is  not  technically  so  successful  as  the  frothing  process. 
If  the  amount  of  air  is  excessive  relatively  to  the  ore,  the  number 
of  bubbles  will  be  so  great  that  the  ore  particles  cannot  coat  them 
sufficiently  to  armor-plate  them ;  the  bubbles  will  therefore  be  in- 
stable.  This  principle  is  involved  in  the  Callow  process.  Air 
is  introduced  in  fine  bubbles  at  the  bottom  of  the  cell  These 


EMULSIONS  AND  FOAMS  273 

bubbles  float  up  the  ore  particles  but  break  as  soon  as  they 
reach  the  surface.  The  ore  particles  must  be  removed  before 
they  settle  back.  We  can  therefore  have  clotted  particles, 
armor-plated  bubbles,  or  fragile  bubbles,  depending  upon  the 
conditions. 

The  flotation  process  will  not  work  with  large  lumps  because 
the  force  of  gravity  would  carry  them  down.  One  of  the  ad- 
vantages of  the  method  is  that  it  will  work  with  finely  ground 
material.  On  the  other  hand  there  is  a  limit.  It  is  customary 
to  say  that  the  ore  goes  into  the  interface  and  the  gangue 
does  not;  but  that  is  not  to  be  taken  too  literally.  Some  gangue 
always  goes  into  the  interface  and  the  percentage  increases  the 
finer  the  gangue.  The  ideal  condition  therefore  is  one  in  which 
the  gangue  is  coarse  relatively  to  the  ore  particles. 

That  the  essential  thing  is  the  adsorption  of  the  particles  at 
the  oil-water  interface  is  shown  by  the  fact  that  the  introduction 
of  saponin  kills  the  flotation  of  the  ore  under  ordinary  conditions 
although  it  increases  the  frothing  tremendously. 

A  carbon  dioxide  froth,  made  from  aluminum  sulphate  and 
sodium  bicarbonate,  is  used  for  putting  out  fires.  It  is  not  suc- 
cessful against  burning  aluminum  powder  for  two  reasons.  The 
aluminum  powder  is  floated  up  by  the  froth  and  consequently 
not  blanketed  by  it.  Also  hot  aluminum  powder  reduces  carbon 
dioxide  to  carbon  monoxide  which  then  burns  in  the  air.  Fortu- 
nately few  fires  are  due  to  aluminum  powder.  The  alumina 
froth  is  said  to  be  stabilized  by  the  addition  of  other  colloids 
such  as  licorice;  but  no  details  have  been  published. 


18 


CHAPTER  X 
NON-AQUEOUS  COLLOIDAL  SOLUTIONS 

Very  little  systematic  work  has  been  done  on  sols  and  gels 
with  liquids  other  than  water.  Graham  replaced  the  water  in 
a  silicic  acid  jelly  by  alcohol,  sulphuric  acid,  nitric  acid,  and 
formic  acid.  Van  Bemmelen  substituted  acetone  for  water,  while 
Bachmann1  put  in  benzene.  Butschli  replaced  the  water  in 
gelatine  and  agar  jellies  by  alcohol,  for  which  could  then  be 
substituted  chloroform,  turpentine  or  xylene. 

We  know  that  the  calcium  and  magnesium  oleates  form  col- 
loidal solutions  in  organic  liquids  and  that  the  so-called  resinates 
do  the  same  thing.  It  is  very  probable,  though  not  proved,  that 
stearin  exists  in  colloidal  solution  in  olive  oil.  The  whole  in- 
dustry of  the  cellulose  nitrates  and  cellulose  acetates  depends 
upon  the  formation  of  colloidal  solutions  in  organic  solvents. 
Pieroni  and  Girardi2  have  studied  some  colloidal  solutions  in 
pyridine.  Water  can  be  emulsified  in  lanolin  and  in  linseed  oil. 
Svedberg3  has  prepared  colloidal  solutions  of  metals  in  ether, 
pentane,  isobutyl  alcohol,  etc.  Schneider4  has  made  alcoholic 
solutions  of.  colloidal  silver  and  finds  that  they  are  coagulated  at 
once  by  isopropyl  alcohol,  normal  and  secondary  butyl  alcohol, 
trimethyl  carbinol,  and  heptyl  alcohol,  but  not  by  propyl  or 
isobutyl  alcohol.  Colloidal  sulphur  may  be  formed  when  sul- 
phur chloride  reacts  with  ethylene.5  Michael6  obtained  col- 
loidal sodium  chloride  without  knowing  it  by  the  interaction  of 
chloracetic  ester  and  sodium  malonic  ester,  while  Paal7  showed 

1  Zeit.  anorg.  Chem.,  03,  165  (1911);  ANDERSON:  Zeit.  phys.  Chem.,  77, 
191  (1914). 

2  Zeit.  Kolloidchemie,  15,  159  (1914). 

3Ber.  deutsch.  chem.  Ges.,  38,  3616  (1905);  39,  1705  (1906). 

4  Ibid.,  26,  1281  U892). 

5  CONANT:  Jour.  Am.  Chem.  Soc.,  42,  585  (1920). 
6Ber.  deutsch.  chem.  Ges.,  38,  3217  (1905). 

7  Ibid.,  39,  1436,  2859  (1906);  41,  51  U908);  42,  277  U909). 

274 


NON-AQUEOUS  COLLOIDAL  SOLUTIONS  275 

that  petroleum  ether  precipitated  the  sodium  chloride,  which 
would  go  again  into  apparent  solution  on  treatment  with  benzene. 
Copper  oleate1  stabilizes  a  colloidal  solution  of  cupric  chloride  in 
benzene. 

It  seems  probable  that  trinitroxylene  (TNX)  is  peptized  by 
trinitrotoluene  (TNT)  for  Marshall2  says  that  when  30-50  parts 
of  TNX  were  suspended  in  70-50  parts  of  TNT,  the  mass  was  suf- 
ficiently fluid  to  be  cast  without  the  use  of  any  extruding  mech- 
anism. The  cooled  castings  appeared  amorphous  and  resembled 
paraffin  in  texture.  The  castings  showed  no  segregation  of  either 
constituent,  were  free  from  air-holes,  were  of  uniform  density,  were 
non-hygroscopic,  and  showed  no  tendency  to  become  oily  or 
leaky  on  storage.  They  could  be  detonated  completely  with 
smaller  primers  of  tetryl  than  could  castings  of  TNT  alone. 

Bechhold3  has  been  able  to  separate  some  of  the  asphalt  out  of 
a  crude  oil  by  ultrafiltration  through  a  toluene-glacial  acetic  acid 
collodion  filter.  Amberger  has  made  colloidal  solutions  of  several 
metals  in  lanolin.  Richardson4  finds  that  asphalt  from  certain 
sources  contains  solid  particles  dispersed  in  the  asphalt.  Lorenz5 
has  shown  that  the  fused  baths,  from  which  we  obtain  sodium, 
aluminum,  and  other  metals,  electrolytically,  tend  to  peptize 
the  molten  metal.  The  amount  of  peptization  or  production  of 
metallic  fog  increases  very  rapidly  with  rising  temperature,  which 
is  one  reason  why  it  is  very  important  to  carry  on  the  electrolysis 
at  as  low  a  temperature  as  possible.  In  the  glass  industry  col- 
loidal solutions  are  very  common,  with  stannic  oxide,  gold,  copper, 
etc.,  as  the  dispersed  phase. 

During  the  war  a  colloidal  fuel  was  developed,  consisting  of 
coal  in  fuel  oil.  Since  the  mixture  is  denser  than  water,  a  fire 
can  be  put  out  by  spraying  with  water,  so  that  the  fire  risk  is 
claimed  to  be  less  than  with  oil  fuel.  Shoe  blacking  is  another 
case  of  a  colloidal  solution  with  a  non-aqueous  solvent.  Acheson 
oildag  is  a  third  variant  of  the  same  general  type. 

1  VON  WEIMARN:  Kolloidchemische  Beihefte,  1,  409  (1910). 

2  Jour.  Ind.  Eng.  Chem.  12,  248  (1920). 
'Colloids  in  Biology  and  Medicine,  11  (1910). 

4  Jour.  Phys.  Chem.,  19,  241  (1915);  ROSINGER:  Zeit.  Kolloidchemie,  15, 
177  (1914). 

6  Van  Bemmelen  Gedenkboek,  395  (1910). 


CHAPTER  XI 
FOG  AND  SMOKE 

FOG 

A  fog  consists  of  drops  of  liquid  suspended  in  a  gas,  usually  air, 
while  a  smoke  has  solid  particles.  To  obtain  a  fog  a  vapor  must 
be  saturated  suddenly,  so  that  condensation  takes  place  more  or 
less  uniformly  throughout  the  mass.  On  a  laboratory  scale  this 
cooling  can  best  be  brought  about  by  an  adiabatic  expansion  or 
by  letting  a  jet  of  steam  emerge  into  a  cooler  atmosphere.  The 
first  method  is  the  more  satisfactory  for  quantitative  measure- 
ments and  the  second  for  qualitative  determinations.  If  the 
supersaturation  is  excessive,  the  liquid  comes  down  as  a  dense  fog 
without  presenting  any  specially  interesting  characteristics.  If 
the  supersaturation  is  slight,  it  is  found  that  no  fog  forms  unless 
" nuclei"  are  present  in  the  air.  This  matter  was  studied  with 
great  thoroughness  by  Coulier1  who  showed  that  there  is  almost 
always  dust  in  the  air  and  that  condensation  takes  place  on  the 
dust  particles.  The  dust  can  be  removed  by  filtering  the  air 
through  cotton.  Ozonized  air  is  active  in  causing  condensation 
and  this  activity  is  not  destroyed  by  filtering  the  air  through  cot- 
ton. Oxygen,  hydrogen,  and  nitrogen  become  active  when 
heated  and  this  activity  is  not  destroyed  by  filtration.  There 
are  therefore  other  types  of  nuclei  besides  solid  particles. 

It  was  found  by  R.  v.  Helmholtz2  that  the  vapors  of  sulphuric 
acid,  hydrochloric  acid,  and  sulphurous  acid  act  as  nuclei.  In  a 
later  paper  R.  v.  Helmholtz  and  Richarz3  advance  the  view  that 
condensation  occurs  frequently  during  chemical  reactions  even 
though  the  reacting  substances  and  reaction  products  do  not  act 
as  nuclei.  They  point  out  that  neither  ozone  nor  oxygen  acts 

1  Jour.  Chem.  Soc.,  29,  186  (1876);  AITKEN:  Trans.  Roy.  Soc.  Edin.,  30, 
337  (1883). 

2  Wied.  Ann.,  27,  508  U886). 
s  Ibid.,  40,  161  (1890). 

276 


FOG  AND  SMOKE  277 

as  a  nucleus  but  that  nuclei  are  present  while  ozone  is  decompos- 
ing. A  similar  phenomenon  was  observed  in  the  case  of  a  num- 
ber of  slow  oxidations  and  reductions.  Hydrochloric  acid  and 
ammonia  are  said  not  to  act  singly  nor  does  ammonium  chloride; 
but  ions  are  formed  during  the  reaction.  This  statement  in 
regard  to  hydrochloric  acid  contradicts  the  previous  statement  as 
to  this  substance  but  there  is  a  difference  of  concentration  and  the 
authors  are  also  willing  to  admit  that  some  ammonia  in  the  air 
may  have  reacted  with  the  acid  in  the  earlier  experiments.  On 
this  statement  of  facts  the  authors  conclude  that  the  reactions 
pass  through  an  intermediate  stage  in  which  ions  are  formed, 
and  that  these  ions  act  as  nuclei.  This  extremely  interesting 
hypothesis  is  put  on  the  defensive  by  the  experiments  of  Engler 
and  Wild1  who  claim  that  solid  substances  are  the  real  nuclei  in 
all  these  cases,  iodine  pentoxide  in  the  oxidation  of  iodine,  an 
oxide  of  phosphorus  in  the  oxidation  of  phosphorus,  ammonium 
chloride,  etc.  Helmholtz  and  Richarz  made  a  good  deal  of  the 
fact  that  the  nuclei  could  not  be  suspended  solids  because  they 
were  not  removed  when  the  gases  were  bubbled  through  water. 
Engler  and  Wild  point  out  that  the  solids  in  the  centre  of  the  gas 
bubbles  do  not  come  in  contact  with  the  water  at  all  and  that  it 
is  a  simple  matter  to  carry  suspended  particles  of  phosphorus 
pentoxide  through  several  wash-bottles  in  spite  of  its  tremend- 
ously hygroscopic  nature.  The  difficulty  of  removing  suspended 
solids  is  shown  by  the  special  precautions  necessary  to  stop  such 
toxic  smokes  as  diphenylchlorarsine. 

Pringal2  considers  that  the  nuclei  attributed  to  ozone  are  really 
oxides  of  nitrogen  and  believes  that  ozone  oxidizes  nitrogen  to 
some  extent.  Bieber3  concedes  that  oxides  of  nitrogen  will 
act  as  nuclei;  but  shows  that  they  are  not  formed  by  the  action 
of  ozone  on  nitrogen,  and  that  the  oxides  of  nitrogen  do  not  form 
the  stable,  blue  cloud  which  is  observed  so  often  when  working 
with  ozone.  He  attributes  this  stable,  blue  cloud  to  hydrogen 
peroxide,  showing  that  the  cloud  is  formed  when  hydrogen  per- 
oxide is  present  and  that  it  does  not  occur  under  conditions 
which  preclude  the  formation  of  hjrdrogen  peroxide.  Thus  we 

1  Ber.  deutsch.  chem.  Ges.,  29,  1929  (1896). 
2Drude's  Ann.,  26,  727  (1908). 
3  Ibid.,  39,  1313  (1912). 


278  APPLIED  COLLOID  CHEMISTRY 

do  not  get  it  with  water  vapor  alone  or  with  water  vapor  plus 
a  gas  other  than  oxygen.  Now  hydrogen  peroxide  can  be  formed 
by  the  action  of  ultra-violet  light  on  water,  especially  on  liquid 
water,  according  to  the  equation  H4O2  =  H2O2  +  H2.  It  may 
also  be  formed  by  the  action  of  ultraviolet  light  on  a  mixture  of 
hydrogen  and  oxygen,1  according  to  the  equation  H2  +  02  = 
H2O2.  This  second  reaction  of  course  plays  no  part  in  the  case 
under  consideration.  Bieber  rules  out  the  first  one  because  no 
hydrogen  is  formed  in  presence  of  ozone,  and  consequently  he 
believes  that  the  reaction  is  H2O  +  O  =  H202,  where  the  oxygen 
atom  is  the  third  one  in  ozone  or  is  set  free  from  oxygen  by  ultra- 
violet light.  While  this  may  be  true,  it  is  not  necessarily  so.  It 
is  quite  as  simple  to  consider  that  the  ozone  acts  as  a  depolarizer 
to  the  nascent  hydrogen  since  it  is  very  improbable  that  the 
action  of  ultraviolet  light  could  give  rise  to  the  reaction  H4O2  + 
Oa  =  H2O2  +  H2O  +  O2,  because  we  know  that  the  intermediate 
product,  electrically  neutral  monatomic  hydrogen,  reacts  with 
oxygen. 

Haber  and  Just2  have  shown  that  when  the  alkali  metals 
react  with  different  gaseous  substances  such  as  water  vapor,  hydro- 
chloric acid,  iodine,  phosgene,  oxygen,  thionyl  chloride,  etc., 
negatively  charged  particles  are  set  free  in  the  reaction  zone  and 
can  be  removed  from  it  by  an  electric  field.  When  the  pressure 
of  the  reacting  gas  is  very  low,  the  negative  particles  pass  out 
from  the  reaction  zone  of  their  own  accord  and  prove  to  be  chiefly 
free  electrons.  In  presence  of  light  there  is  an  enormous  increase 
in  the  emission  of  electrons.  Similar  results  were  obtained  with 
silver,  copper,  and  aluminum  when  heated  or  when  the  pressure 
or  the  gas  was  very  low. 

While  these  experiments  of  Haber  and  Just  prove  conclusively 
that  electrons  are  emitted  during  some  chemical  reactions,  they 
do  not  prove  at  all  that  there  is  always  an  intermediate  formation 
of  ions  in  every  chemical  reaction.  On  the  other  hand,  we  do 
know  that  gaseous  ions,  if  present,  may  act  as  nuclei.  As  Donnan3 
points  out,  the  very  interesting  experiments  of  C.  T.  R.  Wilson4 

1  FISCHER  and  WOLF:  Ber.  deutsch.  chem.  Ges.,  44,  2956  (1911). 
2Zeit.  Elektrochemie,  16,  275  (1910);  JUST:  Ibid.,  20,  483  (1914). 

3  Phil.  Mag.  (6),  8,  306  (1902). 

4  Phil.  Trans.,  189A,  265  (1897);  193A,  289  (1899). 


FOG  AND  SMOKE  279 

on  the  condensation  of  water  vapor  in  the  presence  of  dust-free 
air  and  other  gases,  have  rendered  it  extremely  probable  that  this 
condensation  occurs  around  electrical  nuclei  of  some  sort.  It 
was  found  that  an  adiabatic  expansion  produced  no  condensation 
in  dust-free  air  saturated  with  aqueous  vapor  until  the  expansion 
reached  the  value  of  1.25.  This  expansion  (ratio)  produced  a 
small  shower  of  fine  rain,  and  expansions  ranging  from  the  above 
value  up  to  1.38  produced  small  fine  showers  of  not  much  greater 
density.  At  an  expansion  of  1.38,  however,  the  phenomenon 
changes  in  character,  and  a  more  or  less  dense  fog  is  produced. 
Now  air  or  any  other  gas  exposed  to  Rontgen  rays  becomes  a 
conductor;  and  the  experiments  of  J.  J.  Thomson  and  his  fellow- 
investigators  have  made  it  practically  certain  that  this  conduction 
is  of  a  convective  nature,  due  to  electrically  charged  nuclei  in 
the  gas.  The  importance  of  this  lies  in  the  fact  that  Wilson's 
experiments  showed  that  air  which  was  saturated  with  water 
vapor  and  exposed  to  Rontgen  radiations  gives,  when  expanded 
adiabatically,  no  condensation  before  the  expansion  1.25,  and  a 
dense  fog  instead  of  a  fine  rain  at  that  point.  The  natural  con- 
clusion is  that  the  rain-like  condensation  for  expansions  ranging 
from  1.25  to  1.38  is  due  to  condensation  on  a  comparatively 
small  number  of  electrical  nuclei  either  existing  in  the  gas  or 
produced  by  the  sudden  expansion.  Donnan  was  especially 
interested  in  the  fact  that  here  is  a  "case  of  condensation  of  a 
vapor  of  a  liquid  of  unique  ionizing  power  round  electrical  nuclei. 
It  becomes  a  matter  of  interest  to  compare  with  water  vapor  the 
vapors  of  other  liquids  whose  ionizing  powers  are  either  much 
smaller  or  practically  zero."  He  came  to  the  conclusion  that 
organic  liquids  condensed  on  ions  less  readily  than  does  water.1 
When  a  gas  is  previously  saturated  with  water  vapor,  the  low- 
ering of  the  pressure  necessary  to  produce  condensation  by  adia- 
batic expansion2  is  about  15  cm  for  air,  oxygen,  and  nitrogen,  but 
about  21  cm  for  hydrogen.  Condensation  of  water  takes  place 
more  readily  on  negative  ions  than  on  positive  ions;3  the  reverse 
is  true  for  alcohol.4  In  dust-free  and  ion-free  gases  which  do 

'  Cf.  BARUS:  Phil.  Mag.  (6),  4,  262  (1902). 
2  BIEBER:  Drude's  Ann.,  39,  1313  (1912). 
*  WILSON:  Phil.  Trans.,  193A,  289  (1890). 
4  PRZIBRAM:  Phys.  Zeit.  A.,  8,  561  (1907). 


280  APPLIED  COLLOID  CHEMISTRY 

not  react  with  water  vapor,  the  condensation  takes  place  much 
less  readily,  at  an  eight-fold  supersaturation  according  to  Wilson. 

With  water  vapor  the  expansions  1.25,  1.28,  1.31,  and  1.38 
are  the  values:  (a)  to  start  catching  the  negative  ions;  (6)  to 
catch  all  the  negative  ions;  (c)  to  start  catching  the  positive  ions; 
(d)  to  catch  the  nuclei  of  cloudy  condensation.1 

Since  gaseous  ions  act  as  nuclei2  it  is  not  surprising  that  conden- 
sation may  take  place  when  inert  air  is  exposed  to  cathode  rays, 
Rontgen  rays,  Becquerel  rays  or  ultra-violet  light;  also  when  a 
spark  discharge  or  a  silent  discharge  passes  through  the  air.  The 
ions  given  off  by  heated  metals  may  also  act  as  nuclei. 

Broadly  speaking,  the  nuclei  consist  of  very  hygroscopic  sub- 
stances, solids,  and  ions.3  The  hygroscopic  substances  form 
solutions  having  a  very  low  vapor  pressure  and  the  water  vapor 
is  excessively  supersaturated  when  they  are  present,  consequently 
condensation  takes  place.  Freundlich4  accounts  for  the  behavior 
of  suspended  solids  by  postulating  that  they  have  concave  sur- 
faces on  which  water  would  have  a  lower  vapor  pressure  than  on 
a  convex  surface  and  consequently  would  precipitate  more  readily. 
This  is  very  ingenious  but  carries  with  it  the  corollary  that  solid 
spheres  would  not  act  as  nuclei.  While  this  may  be  true,  it  is 
not  probable.  It  seems  much  simpler  to  consider  that  water 
vapor  is  adsorbed  by  the  suspended  solid,  forming  the  equivalent 
of  a  liquid  surface  on  which  the  further  excess  of  water  vapor 
condenses.  It  has  been  shown  by  J.  J.  Thomson5  that  the  effect 
of  electrification  of  a  drop  of  liquid  is  to  decrease  the  vapor  pres- 
sure of  the  liquid.  Consequently  water  vapor  will  tend  to  pre- 
cipitate on  an  ion  as  a  nucleus,  which  is  what  happens 
experimentally. 

While  fog  usually  consists  of  condensed  water,  there  are  condi- 
tions under  which  special  types  occur.  Thus  Frankland6 
describes  what  he  calls  dry  fog.  Dense  fogs  have  been  noticed 

1  Quoted  from  WILSON  by  OWEN  and  ROBERTS:  Phil.  Mag.  (6),  23,  352 
(1912). 

2  FREUNDLICH:  Kapillarchemie,  294  (1909). 

3  For  a  recent  general  discussion,  see  ROTHMUND:  Monatsheft  fiir  Chemie, 
39,  571  (1918). 

4  Kapillarchemie,  295  (1909). 

*  Application  of  Dynamics  to  Physics  and  Chemistry,  166  (1888). 
6Proc.  Roy.  Soc.,  28,  238  (1879). 


FOG  AND  SMOKE  281 

around  London  when  the  degree  of  saturation  was  only  50-80 
percent.  Frankland  believed  that  this  was  due  to  films  of  oil 
from  coal  smoke  coating  the  drops  of  water  and  retarding  the 
evaporation.  While  Frankland's  explanation  of  the  dry  fogs  is 
probably  the  correct  one,  there  are  two  possibilities  which  are 
not  covered  by  the  experiments.  He  has  not  shown  that  the 
oily  or  tarry  matters  condense  on  the  drops  of  water.  It  is  certain 
that,  under  some  conditions,  the  water  vapor  might  condense  on 
the  oily  drops  as  nuclei,  in  which  case  the  water  would  be  outside 
and  not  inside.  He  has  also  not  considered  the  possibility  of  the 
"  dry  ness"  being  due  to  a  lowering  of  the  vapor  pressure  of  the 
drops  by  the  dissolving  in  them  of  sulphur  dioxide,  sulphuric  acid, 
etc.  While  it  is  not  probable  that  sufficiently  concentrated  solu- 
tions would  be  formed  in  this  way  to  produce  so  great  an  apparent 
dryness,  yet  it  is  very  probable  that  this  is  at  least  a  contributing 
factor. 

At  times  a  special  state  of  things  occurs  at  the  seashore.  If 
the  wind  blows  very  hard  from  the  sea,  a  heavy  surf  springs  up 
and  salt  spray  is  carried  inland.  Ordinarily  this  does  no  harm 
to  vegetation  because  a  wind  of  this  sort  is  usually  accompanied  by 
rain  which  dilutes  the  spray  sufficiently  to  make  it  harmless. 
One  summer  at  Newport,  R.  L,  the  wind  blew  hard  from  the 
south  for  sixty  hours  without  any  rain.  The  result  was  that 
undiluted  salt  spray  was  carried  inland  in  enormous  quantities. 
Everything  was  coated  with  salt;  and  trees,  shrubs,  and  plants 
were  burned  brown  for  nearly  half  a  mile  from  the  shore.  At 
Cirencester  in  England  the  wind  causes  an  average  annual 
deposit  of  thirty-six  pounds  of  sodium  chloride  per  acre.1 

Supercooled  fogs  seem  also  to  occur  at  times.2  On  Ben  Nevis 
the  top  of  the  hill  in  ordinary  weather  is  enveloped  in  drifting 
fog.  When  the  temperature  of  the  air  and  ground  is  below  freez- 
ing, this  fog  deposits  feathery  crystals  of  ice  on  every  surface  that 
obstructs  its  passage.  A  round  post  will  have  an  almost  uniform 
crop  of  crystals  on  its  windward  half,  pointing  so  accurately  to 
windward  that  it  is  possible  to  trace  changes  in  the  direction 
of  the  wind  from  the  successive  layers  of  crystals  lying  at  different 

1  VON  ENGELN:  Am.  Jour.  Sci.  (4)  40,  464  (1915). 

2  POLLOCK  and  COLLIER:  Nature,  31,  216;  OSMOND:  Ibid.  31,  532  (1885); 
MCCONNEL:  Phil.  Mag.  (5)  29,  461  (1890). 


282  APPLIED  COLLOID  CHEMISTRY 

angles.  The  rate  of  growth  varies  with  the  density  of  fog  and  the 
speed  of  the  wind.  A  rough  average  is  half  an  inch  per  hour  and 
two  inches  per  hour  seems  to  be  a  maximum  value.  The  phe- 
nomenon is  what  one  would  expect  from  liquid  particles  cooled 
below  the  freezing  point.  The  same  thing  occurs  on  Mount 
Washington  where  the  crystals  are  known  as  frost  feathers,1 
though  they  are  not  considered  as  resulting  from  supercooled 
fogs.  On  Mount  Washington  the  frost  feathers  are  formed  only 
when  the  wind  is  northerly.  With  a  southerly  wind,  masses  of 
greenish-blue  ice  are. formed,  especially  on  the  telegraph  poles. 

These  supercooled  fogs  are  quite  different  from  the  so-called 
ice  storms  which  occur  once  or  more  every  winter  and  which 
sometimes  do  so  much  damage  to  trees.  In  the  ice  storms  the 
moisture  deposits  as  liquid  water  on  the  branches  and  twigs, 
freezing  there  to  solid  ice.  These  storms  occur  when  the  tem- 
perature is  below  freezing  and  a  warm  rain  or  mist  comes  down 
from  above  without  getting  chilled  to  the  freezing  point  before  it 
strikes  the  trees. 

The  optical  properties  of  suspended  drops  of  water  are  very 
interesting,  the  best  known  case  of  this  being  the  rainbow.2 
When  the  sun  is  shining  on  fog  or  mist,  a  fog  bow3  is  often  seen 
differing  considerably  in  appearance  from  the  ordinary  rainbow — 
far  broader  and,  though  quite  bright,  nearly  colorless.  The  size 
of  the  bow  depends  on  the  size  of  the  drops,  the  diameter  of  the 
bow  being  less  with  very  small  drops4  than  with  large  drops. 
Horizontal  rainbows  are  often  seen  when  the  surface  of  a  lake  is 
covered  with  minute  drops  of  liquid.5 

Under  suitable  circumstances  we  may  see  coronas  surrounding 
the  sun  or  the  moon.6  They  are  due  to  the  interposition  of  small 
spherules  of  water,  which  act  the  part  of  diffracting  obstacles. 
In  order  to  have  the  formation  of  a  well-defined  corona,  it  is 
essential  that  the  particles  be  exclusively  or  preponderatingly  of 
one  size.  By  measurements  of  coronas  it  is  possible  to  infer  the 

1  Mount  Washington  in  Winter,  285  (1871). 

2  MOORE:  Descriptive  Meteorology,  249  (1910). 

3  McCoNNEL:  Phil.  Mag.  (5)  29,  453  (1890). 

4  WOOD:  Physical  Optics,  346  (1911). 

6JuDAY:  U.  S.  Monthly  Weather  Review,  44,  65  (1916);    NAKAMURA: 
Jour.  Meteorological  Soc.  Japan,  36,  1;  OTOBE:  Ibid.  36,  3,  7  (1917). 
6  Encyclopaedia  Britannica,  llth  Ed.  8,  244  (1910). 


FOG  AND  SMOKE  283 

size  of  the  particles  to  which  they  are  due,  an  application  of  con- 
siderable interest  in  the  case  of  natural  coronas,  the  general  rule 
being  that  the  corona  is  larger  the  smaller  the  water  particles. 
Young  employed  this  method  not  only  to  determine  the  diam- 
eters of  cloud  particles  (e.g.,  1/1000  inch);  but  also  those  of 
fibrous  materials  for  which  the  theory  is  analogous. 

Every  one  is  familiar  with  the  colored  coronas  surrounding 
bright  objects,  as  seen  through  a  pair  of  spectacles  freshly  bedewed 
with  moisture.  These  colors  are  due  to  optical  interference  and 
are  entirely  analogous  to  those  seen  through  a  fog.  .  Such  circles 
may  be  seen  around  the  sun  almost  any  day  of  the  year  if  we 
view  it  by  reflection  from  some  poorly  reflecting  surface,  like 
water,  in  order  to  eliminate  the  blinding  glare  of  the  sunshine. 
In  the  case  of  the  largest  corona,  known  as  Bishop's  Ring,  the 
average  diameter  of  the  particles1  of  this  ring  by  diffraction  was 
calculated  to  be  about  0.00015  of  an  inch  (about  4/*). 

The  production  of  coronas  in  the  laboratory  was  observed  by 
Coulier  in  the  paper  referred  to  and  has  been  the  subject  of  study 
for  many  years  by  Barus.2  To  get  the  best  effects  the  drops 
should  be  uniform  in  size. 

Piazzi  Smyth3  reports  that  for  several  days  in  India  and  else- 
where the  disc  of  the  sun  was  blue  or  green  in  the  morning  and 
evening.  He  considers  that  this  was  due  to  a  high  concentration 
of  water  vapor  which  cut  out  the  red  end  of  the  spectrum;  but 
this  does  not  seem  to  be  an  absolutely  satisfactory  explanation. 

The  brilliant  sunset  and  sunrise  colors  are  due  in  part  to 
the  presence  of  suspended  particles  of  water. 4  In  the  lower  half  of 
the  atmosphere  there  are  frequently  larger  particles,  probably  of 
aqueous  vapor,  dust,  and  smoke  that  scatter  the  blue  and  trans- 
mit the  red,  so  that  after  sunset  or  before  sunrise  beams  of  reddish 
light  permeate  the  atmosphere  above  our  heads. 

While  fog  is  due  to  the  production  of  very  small  drops,  rain  is 
due  to  the  formation  of  larger  drops  which  fall  with  perceptible 

1  These  were  solid  particles;  but  formed  a  corona  and  not  a  halo.     A 
corona  is  due  to  diffraction  and  is  red  on  the  outside.     A  halo  is  due  to  re- 
fraction and  is  red  on  the  inside. 

2  Smithsonian  Inst.,  29,  No.  13  (1009);  Carnegie  Inst.  Pub.  No.  62  (1907); 
No.  96  U908). 

'Trans.  Roy.  Soc.  Edin.,  32,  389  (1884). 

4  MOORE:  Descriptive  Meteorology,  245,  255  (1910). 


284 


APPLIED  COLLOID  CHEMISTRY 


speed.  There  is  no  absolute  dividing  line  between  the  two 
because  most  people  would  classify  as  rain  what  a  Scotchman 
might  call  a  mist.  According  to  the  Weather  Bureau,  rain 
appears1  to  begin  when  the  drops  reach  a  diameter  of  0.04  mm. 
The  maximum  rate  of  fall  of  these  drops  is  very  small.  Larger 
drops  fall  faster.  According  to  Lenard2  drops  of  1.28  mm  diam- 
eter have  a  final  velocity  of  4.8  meters  per  second,  those  with  a 
diameter  of  3.49  mm  reach  a  velocity  of  7.37  meters  per  second, 
while  drops  from  4.50-6.36  mm  show  a  practically  constant 
rate  of  about  8  meters  per  second.  This  limiting  value  is  due  to 
the  drops  becoming  deformed,  so  that  they  become  flattened  out, 
instead  of  retaining  the  shape  of  spheres.  They  therefore  offer 
an  increased  resistance  to  the  air  through  which  they  fall.  In 
consequence  of  this  deformation  large  drops  break  up  rapidly  in 
the  air  into  smaller  drops.  Lenard  found  that  drops  of  4  mm 
diameter  were  stable  under  all  conditions,  but  that  drops  of  5.5 
mm  diameter  and  over  could  not  exist  for  more  than  a  few  sec- 
onds after  attaining  their  final  velocity  relatively  to  the  air. 
This  is  in  harmony  with  the  results  of  other  observers.  Wiesner3 
concluded  that  natural  rain  drops  cannot  have  a  diameter  larger 
than  7.2  mm,  while  Hitter  found  that  the  largest  drops  of  natural 
rain  did  not  exceed  6.6  mm  diameter.  An  independent  confirma- 
tion is  found  in  Bentley's  experiments  on  the  size  and  frequency 
of  drops  during  rains  in  northern  Vermont.  His  data  are  given 
in  Table  LI. 

TABLE     LI. — MEASUREMENTS    OF     DIAMETERS     AND     FREQUENCIES     OF 

RAIN  DROPS 


Apparent  size 

Diameter  of 

Frequenc 

y  of  drops 

drops  in  mm 

No.  observed 

Percent 

Very  small 

Under  0  8 

149 

17 

Small  

0.8-1.5 

288 

34 

Medium  . 

1  6-3  5 

254 

29 

Large 

3  4-5  0 

141 

16 

Very  large  

Over  5.0 

1  MOORE:  Descriptive  Meteorology,  205  (1910). 

2  Met.  Zeit.,  21,  249  (1904). 

3  MOORE:  Descriptive  Meteorology,  208  (1910). 


FOG  AND  SMOKE  285 

It  is  possible  that  the  very  smallest  drops  often  escaped 
observation  and  that  the  percentage  of  very  small  drops  should 
be  more  than  17.  In  any  case  the  mean  diameter  is  not  over  1.5 
mm  and  the  instructive  thing  is  to  see  the  very  small  percentage 
of  drops  larger  than  5.0  mm  diameter. 

Simpson1  has  recently  developed  a  very  interesting  theory  in 
regard  to  thunderstorms.  He  considers  the  case  of  a  rapidly 
ascending  current  of  air  which  spreads  out  and  slows  up  above  a 
certain  height.  Large  rain  drops  will  fall  through  this  air  current 
until  they  reach  a  point  where  they  are  broken  into  smaller  drops 
and  are  then  carried  to  the  upper  and  colder  levels  where  they 
grow  again  and  repeat  the  cycle.  When  the  drops  break  up  they 
become  charged  positively,  while  the  negative  ions  are  carried 
up  more  rapidly  by  the  air  and  are  finally  caught  by  cloud  parti- 
cles at  some  higher  level.  On  this  hypothesis  positively  charged 
rain  should  fall  in  bursts  in  the  center  of  the  thunderstorm  while 
negatively  charged  rain  should  fall  in  the  lulls  and  on  the  out- 
skirts of  the  thunderstorm.  This  actually  happens.  Simpson 
satisfied  himself  that  the  electricity  generated  by  the  breaking  up 
of  the  falling  drops  might  easily  account  for  the  gradient  of  30,000 
volts/cm  necessary  for  lightning.  No  calculations  were  made 
by  Simpson  as  to  the  possibility  of  duplicating  these  phenomena  in 
the  laboratory  by  a  vertical  blower.  If  it  were  feasible  to  have  a 
synthetic  thunderstorm,  this  would  be  of  great  value  in  the  study 
of  meteorological  phenomena. 

It  is  interesting  to  note  that  while  drops  become  charged  posi- 
tively on  splashing,  water  vapor  condenses  more  readily  on 
negative  ions2  than  on  positive  ones  and  that  liquid  drops  take 
up  negative  ions  more  readily  than  positive  ones.3 

The  precipitation  of  a  fog  or  mist  is  an  important  problem; 
but  can  best  be  taken  up  in  connection  with  the  precipitation  of 
smoke  or  dust  because  the  technical  methods  are  as  yet  the  same 
in  the  two  cases.  Reference  may,  however,  be  made  here  to  a 
process  devised  by  Pelouze  and  Audouin,4  who  force  the  mist 

1  Phil.  Trans.,  209A,  379  (1909);  HUMPHREYS:  Jour.  Franklin  Inst.,  178» 
751  (1914);  Phys.  Rev.  (2),  6,  516  (1915). 

2  WILSON:  Proc.  Roy.  Soc.,  64,  127  (1898). 

3  SCHMAUSS:  Drude's  Ann.,  9,  224  (1902);  SEELIGEE:  Ibid.,  31,  510;  33,  431 
(1910);  Lenhardt:  Ibid.,  42,  45  (1913). 

4  MOORE:  Descriptive  Meteorology,  205  (1910). 


286  APPLIED  COLLOID  CHEMISTRY 

through  a  series  of  small  apertures  and  cause  the  streams  to 
impinge  on  a  flat  surface  placed  opposite.  They  consider  that 
by  this  means  the  drops  are  brought  into  contact  with  one  another 
and  therefore  coalesce  to  larger  drops,  which  latter  adhere  to  the 
surface  against  which  they  are  driven.  The  method  has  been 
used  for  the  separation  of  tar,  carbon  bisulphide,  etc.,  from  coal 
gas.  It  is  interesting  as  being  the  mechanical  analogue  of  the 
Cottrell  electrical  method. 

SMOKE 

As  an  intermediate  step  between  fog  and  smoke  we  have  the 
precipitation  of  water  vapor  in  solid  form  as  snow,  sleet  or  hail. 
When  the  dew  point  is  decidedly  below  freezing,1  water  vapor  con- 
denses directly  to  crystals  of  ice,  ice  needles,  or  snow  flakes. 
When  supercooled  rain  drops  strike  solid  substances  they  cover 
these  with  a  coating  of  transparent  ice.  When  snow  crystals  and 
supercooled  rain  drops  come  in  contact  we  get  sleet,  and  sleet 
changes  to  hail  when  the  supercooled  drops  precipitate  in  the  form 
of  sheaths  of  ice.  In  order  that  hail  stones  should  grow  to  any 
appreciable  size,  they  must  be  carried  up  and  down  a  number  of 
times.  Even  in  midsummer  the  temperatures  six  kilometers 
above  the  earth's  surface  will  be  at  least  —26°  and  may  easily  be 
—  40°.  The  reason  that  we  get  hail  chiefly  in  summer  is  because 
then  we  have  the  hot  air  of  the  earth's  surface  giving  a  sharp 
temperature  gradient  and  therefore  a  rapid  upward  sweep.  The 
warm  air  may  contain  a  great  deal  of  water  vapor  which  permits 
the  precipitation  of  large  amounts  of  ice.  In  winter  the  vertical 
temperature  gradient  is  less  and  also  the  amount  of  water  vapor 
in  the  air.  Consequently  we  get  no  real  hail,  especially  far 
from  the  sea. 

Hail  stones2  can  only  form  if  they  are  supported  during  forma- 
tion by  strong  ascending  currents.  Further,  the  structure  of  a 
hail  stone  indicates  that  it  is  often  carried  up  and  down  past  the 
zero  isothermal.  Now  a  current  of  air  sufficiently  strong  just 
to  support  a  hail  stone  as  big  as  a  pea  would  be  more  than  suffi- 
cient to  carry  up  the  water  it  condenses  within  itself;  hence  hail 
stones  would  always  have  a  greater  downward  velocity  relative 

1  MOORE:  Descriptive  Meteorology,  205  (1910). 

2  SIMPSON:  Phil.  Trans.,  209A,  412  (1909). 


FOG  AND  SMOKE  287 

to  the  ascending  current  than  the  water  in  the  current  and  there 
would  be  a  large  amount  of  splashing  between  the  two.  There 
would  be,  consequently,  a  much  greater  amount  of  separation  of 
electricity  than  would  have  taken  place  without  the  hail  stones, 
and  this  might  very  well  account  for  the  great  violence  of  the 
electrical  discharges  in  hail  storms. 

Tolman1  and  others  have  devised  a  form  of  ultramicroscope 
which  they  call  a  Tyndallmeter  for  the  quantitative  study  of 
smokes  and  have  determined  the  relation  between  the  intensity 
of  the  Tyndall  beam  and  the  size  of  the  particle,  making  use  also 
of  silica  suspensions  as  standards.  It  has  been  shown  by  Lord 
Rayleigh  on  theoretical  grounds  that,  when  the  particles  are  very 
small  compared  with  the  wave-length  of  light,  the  intensity  of 
the  Tyndall  beam  should  be  proportional  to  the  number  of  par- 
ticles per  unit  volume  and  to  the  sixth  power  of  their  diameter. 
For  larger  particles  Tolman  considers  that  the  intensity  would  be 
proportional  to  the  area  of  reflecting  surface,  in  other  words  to 
the  square  of  the  diameter.  If  we  define  the  concentration,  C, 
as  the  mass  per  unit  volume  irrespective  of  the  number  of  par- 
ticles, we  get  two  equations: 

T  =  RCd3  for  very  small  particles. , 
T  =  RiC/d  for  large  particles. 

For  very  small  particles,  the  Tyndallmeter  reading,  T,  will  in- 
crease with  increasing  diameter,  whereas  it  will  decrease  for  large 
particles.  The  results  show  that,  in  general,  increased  subdi- 
vision means  an  increased  Tyndallmeter  reading.  The  observa- 
tions of  Mecklenburg2  on  sulphur  suspensions  agree  well  with 
Rayleigh's  formula  for  diameters  from  5-840/i/u;  whereas  some  of 
the  smokes  show  the  opposite  result  for  particles  smaller  than 
the  largest  of  these.  Mecklenburg's  work  will  have  to  be  re- 
peated before  the  discrepancy  can  be  explained. 

Unless  subjected  to  special  treatment,  the  air  always  contains 
dust.  The  air  of  large  cities  invariably  shows  hundreds  of  thou- 
sands of  dust-motes  per  cubic  centimeter,3  that  of  the  village  or 
town  thousands,  and  that  of  the  open  country  at  least  hundreds. 

1  Jour.  Am.  Chem.  Soc.,  41,  297,  300,  304,  575  (1919). 

2  Zeit.  Kolloidchemie,  16,  97  (1915). 

3  MOORE:  Descriptive  Meteorology,  30  (1910). 


288  APPLIED  COLLOID  CHEMISTRY 

The  dust  may  be  soil1  from  the  fields  and  roads,  particles  of  animal 
or  plant  origin,  including  pollen,  products  of  combustion,  salt 
from  the  spray  of  the  ocean,  volcanic  dust,  or  cosmic  dust.  The 
dust  from  the  eruption  of  Krakatoa  was  shot  high  into  the  air 
and  was  carried  entirely  round  the  world,  falling  on  the  decks 
of  ships  and  on  various  parts  of  the  earth.  Owing  to  the  enor- 
mous distances  to  which  volcanic  dust  may  be  carried,  it  is  nec- 
essary to  have  some  definite  test  before  we  can  say  that  a  given 
sample  of  dust  is  really  cosmic  dust  coming  from  meteors  which 
have  burned  up  in  passing  through  the  earth's  atmosphere.  This 
matter  has  been  studied  by  Hartley  and  Ramage,2  who  collected 
numerous  samples  of  dust  in  the  neighborhood  of  Dublin  and  ex- 
amined them  spectroscopically  as  well  as  other  samples  furnished 
by  friends.3  "The  specimens  of  soot  required  no  preliminary 
treatment  before  being  burnt,  and  the  analysis  of  each  is  given 
in  the  tabular  statement  only;  but  the  different  kinds  of  volcanic 
dust  and  flue  dust  were  dissolved  and  the  silica  removed,  after 
which  the  bases  were  separated  into  groups,  and  the  spectra  of 
the  groups  photographed."  The  data  for  the  bases  are  given 
in  Table  LII,  italics  being  used  when  only  a  trace  of  the  metal  is 
present.  The  relative  strength  of  the  lines,  as  seen  by  comparing 
the  different  spectra,  is,  in  some  instances,  indicated  in  the  table 
by  suffixes,  the  number  1  indicating  the  weakest  line  and  10  the 
strongest. 

Hartley  and  Ramage  drew  the  following  conclusions: 

(1)  The  presence  of  nickel,  as  shown  by  the  examination  of  soot, 
is  not  positive  evidence  that  the  dust  from  the  clouds  comes  from 
other  than  a  terrestrial  source. 

(2)  The  dust  which  feU  on  the  16th  and  17th  of  November, 
1897,  with  its  regularity  in    composition  and  its  similarity  to 
meteorites,  being  magnetic,  also  its  comparative  freedom  from 
extraneous  matter,  exhibits  properties  which  are  quite  in  favor  of 
its  cosmic  origin.     Moreover,  its  composition  is  totally  unlike  that 
of  volcanic  dust  and  flue  dust  from  various  chemical  and  metal- 
lurgical works.     This  dust  for  the  most  part  fell  on  a  perfectly 

!See  EHRENBERG:  Die  Bodenkolloide,  209  (1915). 
2Proc.  Roy.  Soc.,  68,  97  (1901). 

3  Data  on  the  composition  of  soot  have  been  published  recently  by  COHEN 
and  HUSTON:  Jour.  Soc.  Chem.  Ind.,  32,  1360  (1911). 


FOG  AND  SMOKE 


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290  APPLIED  COLLOID  CHEMISTRY 

calm  fine  night,  and  there  was  no  rain  for  twenty-four  hours  or 
more  afterwards. 

"We  beg  to  draw  attention  once  more  to  the  very  wide  dis- 
tribution of  gallium  in  minute  proportions;  it  occurs  in  all  alu- 
minous minerals,  flue  dust  of  every  kind,  soot  and  atmospheric 
dust,  also  in  a  great  variety  of  iron  ores.  Bauxite  contains  it 
in  larger  proportion  than  any  other  mineral,  but  the  quantity 
even  in  this  substance  is  very  small.  We  have  hopes  of  finding 
it  concentrated  in  some  mineral,  as  thallium,  caesium,  germanium, 
and  indium  are.  Indium  and  thallium,  the  other  members  of 
the  same  group  of  elements,  are  found  in  blende  and  pyrites,  and 
accordingly  we  might  expect  gallium  to  occur  in  a  concentrated 
state  in  a  sulphide,  arsenite,  or  similar  compound.  Judging, 
however,  from  its  analogy  with  aluminum,  there  does  not  seem 
to  be  much  probability  of  this." 

Unfortunately  Hartley  and  Ramage  do  not  tell  us  exactly  how 
to  identify  cosmic  dust  and  it  is  not  an  easy  matter  to  formulate 
a  rule  from  the  data.  The  first  four  samples  of  dust  differ  from 
the  volcanic  dust  in  not  containing  magnesia  or  strontium;  they 
differ  from  the  flue  dust  and  the  soot  from  chimneys  in  not  con- 
taining rubidium  and  in  containing  more  nickel.  If  we  are  to 
take  only  the  dust  which  fell  on  November  16  and  17,  1897,  as 
of  cosmic  origin,  the  only  difference  is  the  trace  of  chromium.  The 
situation  is  complicated  farther  by  the  fact  that  the  meteoric 
stones  all  contain  magnesia  and  chromium.  It  is  not  clear  at 
all  why  these  should  have  disappeared  in  the  alleged  cosmic  dust. 
So  far  as  one  can  formulate  a  conclusion  from  this  work,  it  seems 
to  be  that  dust  is  of  cosmic  origin  when  it  contains  sodium,  po- 
tassium, copper,  silver,  calcium,  gallium,  thallium,  iron,  nickel, 
manganese,  chromium,  and  lead;  but  no  rubidium,  strontium  or 
magnesia.  The  dust  must  also  be  magnetic,  this  last  probably 
being  the  most  important  criterion  of  all  though  it  does  not 
appear  in  the  table. 

Preliminary  work  on  the  precipitation  of  smoke  and  dust  was 
done  by  Nahrwold1  and  by  Aitken,2  the  latter  of  whom  showed 
that  tobacco  smoke  could  be  precipitated  quantitatively  by  pass- 
ing it  through  a  hot-cold  tube,  the  smoke  condensing  on  the  cold 

1  Wied.  Ann.,  6,  460  (1878). 

2  Nature,  29,  322  (1884);  Trans.  Roy.  Soc.  Edin.,  32,  239  (1884). 


FOG  AND  SMOKE  291 

tube.  The  film  of  condensed  air  round  the  particles  has  a  lower 
surface  tension  on  the  hot  side  than  on  the  cold  side  and  conse- 
quently flows  round  the  particles  from  the  cold  side  to  the  hot 
side,  causing  the  particles  to  move  in  the  opposie  dirtection  or 
along  the  gradient  of  falling  temperature.  The  temperature 
gradient  has  to  be  pretty  steep  to  produce  a  marked  effect,  say 
100°/cm,  so  that  the  method  has  never  been  developed  as  a 
practical  means  of  removing  smoke.  A  slight  movement  has 
been  observed  by  Wells  and  Gerke1  due  to  the  heat  from  the 
source  of  light  used  in  the  ultramicroscopic  examination  of  smokes. 

Lodge2  tried  the  effect  of  electrification  on  a  mass  of  smoke. 
With  potentials  of  one  hundred  volts  very  little  effect  could  be 
detected.  When  the  potential  rose  to  a  few  thousand  volts  and 
a  brush  discharge  began  to  be  possible,  the  smoke  agglomerated 
and  settled  very  rapidly.  The  theory  of  this  has  never  been 
worked  out.  In  ordinary  smokes  about  30  percent  of  the 
particles  are  charged  electrically.  It  may  be  that  the  alternating 
current  reverses  the  sign  of  the  charge  periodically  and  that  the 
particles  agglomerate  when  they  are  electrically  neutral  or  it  may 
be  that  it  causes  the  charged  particles  to  collide  with  the  un- 
charged ones.  In  connection  with  this  it  would  be  interesting  to 
determine  the  effect  of  a  high-voltage  alternating  current  on  the 
stability  of  a  suspension  in  a  practically  non-conducting  liquid. 

The  general  principle  of  electrical  precipitation  of  suspended 
particles  was  patented  by  A.  O.  Walker  of  the  Dee  Bank  Lead 
Works.  In  1885  an  apparatus  was  installed  consisting  of  a  system 
of  metallic  points  situated  in  the  flue  from  the  lead  furnaces  and 
excited  from  two  Wimshurst  machines  with  glass  plates  five  feet 
in  diameter,  each  machine  being  driven  by  a  one-horse  power 
steam  engine.  The  process  seems  not  to  have  been  a  commercial 
success.  It  is  only  through  the  work  of  Cottrell3  that  the  elec- 
trical precipitation  of  suspended  particles  has  been  put  on  a  com- 
mercial basis.  Cottrell  believed  that  in  the  case  of  the  large 
volumes  of  rapidly  moving  gases  in  smelter  flues  the  agglomerat- 
ing and  settling  process  was  too  slow  even  though  the  flues  were 
expanded  into  as  large  dust  chambers  as  were  commercially 

1  Jour.  Am.  Chem.  Soc.,  41,  325  (1919). 

a  Phil.  Mag.  (3),  17,  214  (1884);  Jour.  Soc.  Chem.  Ind.,  6,  572  (1886). 

3  Jour.  Ind.  Eng.  Chem.,  3,  542  (1911). 


292  APPLIED  COLLOID  CHEMISTRY 

feasible.  It  is  in  such  cases  that  direct  current  methods  have  been 
particularly  important. 

If  a  needle  point  is  connected  to  one  side  of  a  high  potential 
direct  current  line  opposite  to  a  flat  plate  connected  to  the  other 
side  of  the  line  the  air  space  between  becomes  highly  charged  with 
electricity  of  the  same  sign  as  the  needle  point,  irrespective  of 
whether  this  is  positive  or  negative,  and  any  insulated  body 
brought  into  this  space  instantly  receives  a  charge  of  the  same 
sign.  If  this  body  is  free  to  move,  as  in  the  case  of  a  floating 
particle,  it  will  be  attracted  to  the  plate  of  opposite  charge  and 
will  move  at  a  rate  proportional  to  its  charge  and  the  potential 
between  the  point  and  the  plate.  Even  if  there  are  no  suspended 
particles  the  gas  molecules  themselves  undergo  this  same  process, 
as  is  evidenced  by  a  strong  wind  from  the  point  to  the  plate  even 
in  perfectly  transparent  gases.  The  old  familiar  experiment  of 
blowing  out  a  candle  flame  by  presenting  it  to  a  charged  point, 
is  simply  another  illustration  of  the  same  phenomena. 

The  procedure  actually  used  to  get  direct  current  consisted  in 
transforming  the  alternating  current  from  an  ordinary  lighting 
or  power  circuit  up  to  some  20,000  or  30,000  volts  and  then 
commutating  this  high  potential  current  into  an  intermittent 
direct  current  by  means  of  a  special  rotary  contact  maker  driven 
by  a  synchronous  motor.  This  direct  current  is  applied  to  a 
system  of  electrodes  in  the  flue  carrying  the  gases  to  be  treated. 
While  any  smooth  conducting  surface  worked  well  for  the  plate 
electrodes,  it  was  difficult  to  find  anything  suitable  for  the  point 
electrodes  when  working  on  a  commercial  scale.  The  problem 
was  solved  by  using  asbestos  or  mica,  the  fine  filaments  of  the 
one  and  the  scales  of  the  other  supplying  the  discharge  points  or 
the  edges  of  the  excessive  fineness  required.  These  materials  are 
twisted  up  with  wires  or  otherwise  fastened  to  suitable  metallic 
supports  to  form  the  discharge  electrodes  in  such  wise  that  the 
current  has  to  pass  only  a  short  distance  by  surface  leakage  over 
them,  the  slight  deposit  of  moisture  or  acid  fume,  naturally  set- 
tling on  them,  serving  to  effect  the  conduction.  If  the  condition 
of  the  gases  does  not  supply  such  coating  sufficiently,  a  special 
treatment  of  the  material  before  being  placed  in  the  flue  is 
resorted  to. 

The  process  was  installed  successfully  at  the  contact  sulphuric 


FOG  AND  SMOKE  293 

acid  plant  of  the  Hercules  Works  of  the  E.  I.  du  Pont  de  Nemours 
Powder  Company  at  Pinole  on  San  Francisco  Bay;  at  the  refining 
plant  and  at  the  pyrites  and  matte  roasters  of  the  Selby  Smelting 
and  Lead  Company;  and  at  the  Balaklala  smelter. 

In  the  case  of  smelting  plants  the  process  is  not  a  universal 
panacea  because  it  does  not  remove  gases  and  the  farmers  object 
to  sulphur  dioxide  about  as  much  as  they  do  to  sulphuric  acid, 
arsenic,  etc.  In  cases  where  suspended  solids  are  the  only  source 
of  trouble,  the  process  is  capable  of  eliminating  the  whole  trouble. 
This  was  shown  at  Riverside  in  California,  where  there  is  a  cement 
plant  in  the  neighborhood  of  orange  groves  and  the  dust  of  course 
settles  on  the  leaves.1  A  rotary  kiln  100  feet  long  and  7  feet  in 
diameter,  oil-fired,  will  yield  about  50,000  cubic  feet  of  stack 
gases  per  minute.  The  gases  above  the  combustion  zone  in  the 
stack  have  a  temperature  of  450°C.  and  carry  dust  aggregating 
between  four  and  five  tons  per  twenty-four  hours.  With  these 
dry  gases  it  was  possible  to  use  steel  construction  without  cooling 
the  gases.  The  installation  was  placed  at  the  top  of  the  stacks  80 
feet  up.  It  is  probable  that  a  considerable  portion  of  the  needs  of 
the  country  for  potash  can  be  met  by  utilizing  the  dust  from  the 
cement  plants. 

In  the  Washoe  reduction  works  of  the  Anaconda  Copper  Mining 
Company  the  Cottrell  process  has  been  introduced  on  so  large 
a  scale2  that  the  point  electrodes  consist  of  one  hundred  and 
eleven  miles  of  chains. 

The  Cottrell  process  has  proved  a  great  success  commercially 
and  new  uses  for  it  are  continually  being  found.  A  modification 
of  the  apparatus  has  recently  been  made  for  the  study  of  toxic 
smokes  in  the  laboratory.3  The  apparatus  consisted  essentially 
of  a  central  wire  cathode  surrounded  by  a  cylindrical  anode  of 
aluminum  foil  0.001  inch  in  thickness.  An  alternating  current 
running  up  to  15,000  volts  was  rectified  by  means  of  a  General 
Electrical  Company  "Kenotron."  Complete  precipitation 
could  be  obtained  even  when  smoke  was  drawn  through  the  ap- 
paratus at  a  rate  of  about  five  liters  per  minute.  The  anode 

1  SCHMIDT:  Jour.  Ind.  Eng.  Chem.,  4,  719  (1912). 

2  CHANDLER:  Jour.  Ind.  Eng.  Chem.  11,  138  (1919). 

3  TOLMAN,  REYERSON,  BROOKS  and  SMYTH:  Jour.  Am.  Chem.,  Soc.,  41, 
587  (1919). 


294  APPLIED  COLLOID  CHEMISTRY 

could  be  removed  and  weighed,  thus  giving  a  satisfactory  method 
of  determining  the  concentration  of  any  smoke.  Examination  of 
the  precipitate  microscopically  makes  it  possible  to  tell  whether 
one  is  really  dealing  with  a  smoke  or  with  a  fog. 

When  dust  is  blown  about  by  the  wind,  it  becoms  electrified.1 
In  South  Africa  at  Bloemfontein  at  an  elevation  of  4500  feet,  the 
normal  fine  weather  charge  of  the  air  is  positive  and  seldom  ex- 
ceeds a  maximum  value  of  200  volts  per  meter.  During  a  dust 
storm  the  sign  of  the  charge  changes  and  the  value  may  exceed 
500  volts  per  meter.  A  so-called  dust  devil  or  whirlwind  which 
carries  a  column  of  fine  sand  up  two  or  three  hundred  feet  in  the 
air  will  affect  an  instrument  two  miles  away,  reversing  the  charge. 
This  is  because  the  sand  particles  are  charged  positively.  In 
England  a  cloud  of  dust  increases  the  positive  charge  in  the  air 
instead  of  decreasing  it.  This  is  because  the  dust  in  England  is 
usually  calcareous  and  acquires  a  negative  charge.  It  is  also 
known  that  red  lead  acquires  a  positive  charge  when  blown  into 
the  air  and  sulphur  a  negative  one. 

From  the  experiments  that  he  made,  Rudge  deduced  four 
generalizations.2  Non-metallic  elements  give  positively  charged 
clouds  when  the  finely-divided  solid  material  is  blown  into  a 
cloud  by  a  current  of  air.  Metallic  elements  give  negatively 
charged  clouds  when  the  finely  divided  material  is  blown  into  a 
cloud  by  a  current  of  air.  Solid  acid-forming  oxides  give  posi- 
tively charged  clouds,  and  basic  oxides  negatively  charged  clouds. 
In  the  case  of  salts  the  charge  apparently  depends  on  the  relative 
strength  of  adsorption  of  positive  and  negative  ions.  The  whole 
subject  of  frictional  electricity  should  be  gone  over  from  the  view- 
point of  adsorption  of  ions.  If  this  were  done,  it  would  undoubt- 
edly bring  some  order  out  of  the  present  chaos.  Guttmann3 
gives  some  data  on  electrical  disturbances  in  powder  mills. 

I  am  indebted  to  Dr.  Reid  Hunt  for  information  in  regard  to 
another  difference  between  siliceous  and  calcareous  dusts.  The 
flint  paricltes  in  South  Africa  are  not  rounded  and  the  sharp 
edges  of  the  dust  wound  the  lungs  and  permit  the  tuberculosis 
bacteria  to  enter,  whereas  there  is  not  the  same  danger  in  the 

1  RUDGE:  Phil.  Mag.  (6),  23,  852  (1912);  26,  481  (1913). 
2Cf.  KNOBLAUCH:  Zeit.  phys.  Chem.,  39,  225  (1902). 
3  Jour.  Soc,  Chem.  Ind.,  8,  60  (1889). 


FOG  AND  SMOKE  295 

case  of  calcareous  dusts.  If  dried  rhubarb  is  ground  up,  the  cal- 
cium oxalate  forms  a  dust  composed  of  needles  exceedingly  irritat- 
ing to  the  lungs. 

The  optical  phenomena  due  to  suspended  solids  are  quite  as 
interesitng  as  those  due  to  suspended  drops  of  liquid.  The  Tyn- 
dall  effect,  or  the  scattering  of  a  convergent  or  parallel  beam  by 
suspended  particles,  has  been  referred  to  in  connection  with  the 
ultramicroscope;  and  in  the  discussion  of  the  color  of  colloids  it 
was  pointed  out  that  the  blue  color  of  skimmed  milk  is  due  to  the 
fact  that  the  blue  rays  are  scattered  more  than  the  red.  The 
blue  of  cigar  smoke  is  a  similar  phenomenon  and  Lord  Ray- 
leigh1  has  shown  that  the  blue  of  the  sky  can  be  accounted  for 
very  well  as  due  to  scattering  by  particles  of  water,  ice,  or  dust. 
He  even  claims  that  the  molecules  of  oxygen  and  nitrogen  are 
large  enough  in  themselves  to  give  rise  to  a  blue  sky  even  if  the 
air  contained  no  suspended  matter  in  the  ordinary  sense  of  the 
word.  From  his  formula  he  deduces  the  value  of  the  Avogadro 
constant,  the  number  of  molecules  per  gram  molecule.2  Without 
being  able  to  point  to  any  error  in  Lord  Rayleigh's  mahtematics, 
it  seems  to  me  that  his  assumption  in  regard  to  the  behavior  of 
oxygen  and  nitrogen  molecules  is  probably  wrong.  The  molec- 
ular weight  of  oxygen  is  so  low  that  practically  all  solutions  and 
gaseous  mixtures  would  be  blue  by  reflecetd  light  and  red  by  trans- 
mitted light  if  Rayleigh  were  right. 

Nichols3  is  by  no  means  certain  that  one  can  account  quanit- 
tatively  for  all  the  phenomena  of  the  blue  sky  on  the  basis  of 
solid  or  liquid  particles.  The  distribution  of  intensities  seems 
not  to  be  exactly  that  required  by  Rayleigh's  formula.  The 
best  explanation  of  the  discrepancy  is  either  that  the  air  itself 
absorbs  blue  to  some  extent  or  that  some  variable  constituent  of 
the  air  does,  such  as  ozone,  for  instance. 

Many  of  the  color  effects  in  the  sky  apart  from  the  blue  are  due 
to  the  presence  of  solids.  Moore4  says  that  if  it  were  not  for 
dust-motes  there  would  be  a  different  and  less  brilliant  twilight. 
The  bending  or  refraction  of  light,  as  the  sun's  rays  pass  obliquely 

1  Phil.  Mag.  (4)  41,  107,  274,  447  (1877);  (5)  12,  81  (1881);  47,  375  (1899). 
2Cf.  PERRIN:  Die  Atome,  129  (1914). 

3  Phys.  Rev.,  26,  497  (1908). 

4  Descriptive  Meteorology,  32,  254,  245  (1910). 


296  APPLIED  COLLOID  CHEMISTRY 

from  the  etjjer,  at  sunrise,  or  at  sunset,  into  the  optically  denser 
medium  of  the  air,  displaces  the  apparent  position  of  the  sun, 
elevating  it  by  an  amount  about  equal  to  its  apparent  diameter, 
so  that  one  may  still  see  it  and  receive  its  light  directly  when 
geometrically  it  is  entirely  below  the  horizon.  A  little  later  in 
the  evening  and  its  rays  fall  upon  the  upper  air  too  obliquely  to 
be  bent  down  to  the  earth  by  refraction,  but  darkness  does  not 
yet  ensue,  for  the  rays  are  scattered  by  the  molecules  of  gases 
and  the  dust-motes  and  sent  downward  from  particle  to  particle, 
resulting  in  a  soft  shimmering  light  that  fades  away  almost 
imperceptibly,  and  which  may  last  for  hours  in  higher  latitudes, 
because  of  the  obliqueness  there  of  the  sun's  path  to  the  horizon. 

When  the  harmattan  blows  on  the  west  coast  of  Africa,  the 
sky  has  a  chalky-white  tint  and  the  air  is  very  dry.  The  whitish 
tint  is  due  to  the  presence  of  innumerable  fragments  of  micro- 
scopic diatoms  or  siliceous  shells.  The  whitish  color  of  the  haze 
must  be  attributed  to  the  reflection  of  light  from  their  surfaces. 
A  similar  white  haze  occurs  in  air  that  is  full  of  grains  of  pollen  or 
.  fine  crystals  of  snow,  or  almost  any  other  kind  of  small  particles. 

"An  unusual  illustration  of  selective  absorption  was  given  on 
the  occasion  of  the  most  violent  volcanic  eruption  of  historic 
times — that  of  Krakatoa — on  August  27,  1883,  when  an  immense 
volume  of  dust  and  aqueous  vapor  was  thrown  to  a  great  height 
in  the  atmosphere  above  the  Straits  of  Sunda.  The  antitrades 
spread  this  over  the  northern  hemisphere,  and  winds  higher  up  in 
the  tropics  carried  it  -westward  round  the  world  and  formed  a 
layer  of  from  5  to  15  miles  above  the  earth's  surface,  consisting 
undoubtedly  of  minute  particles.  After  sunset,  and  by  virtue 
of  the  diffusion  of  red  light  by  this  layer  of  particles,  the  whole 
western  sky,  even  to  near  the  zenith,  glared  with  a  lurid  red  as 
though  lighted  up  from  some  great  and  distant  fire.  These 
remarkable  sunsets  continued,  diminishing  slowly  in  brilliance, 
through  the  years  1884  and  1885  in  temperate  latitudes,  while 
their  northern  limit  advanced  slowly  toward  the  pole,  showing 
that  minute  particles  of  solid  matter  may  float  for  years  in  the 
high  upper  atmosphere,  so  long  as  slowly  rising  currents  buoy 
them  up.  Isolated  clouds  of  such  material  may  always  be 
floating  in  the  upper  atmosphere  and  may  give  rise  to  the  nocti- 
lucent  or  night-shining  clouds  that  are  occasionally  observed  at 


FOG  AND  SMOKE  297 

great  altitudes  when  the  observer  looks  northward  in  mid- 
summer." 

The  tails  of  comets  seem  to  consist  almost  entirely  of  colloidal 
particles.1  The  great  comet  of  1882  was  invisible  against  the 
solar  disc,  a  position  which  corresponds  to  attempted  observation 
of  colloidal  particles  in  the  ordinary  microscope  against  a  lumi- 
nous background.  It  became  visible  again  after  passing  beyond 
the  sun's  disc,  a  position  correspondng  to  successful  observation 
of  the  same  colloidal  particles  in  the  ultramicroscope  against  a 
dark  background,  the  eye  of  the  observer  being  protected  from 
the  source  of  illumination.  The  streaming  of  the  tail  of  a  comet 
away  from  the  sun  may  be  due  to  the  ionization  of  the  colloidal 
particles,  and  their  consequent  electrical  repulsion,  it  may  be  a 
thermal  effect,  or  it  may  be  due  to  the  pressure  of  light  as  was 
pointed  out  by  Maxwell.  The  intensity  of  the  action  of  the 
sun's  rays  on  a  particle  depends  on  its  surface  which  varies  as 
the  square  of  the  diameter,  while  the  force  of  gravity  tending  to 
carry  the  particle  to  the  sun  is  proportional  to  the  mass  of  the 
particle  and  this  varies  as  the  cube  of  the  diameter.  In  the 
case  of  a  particle  having  a  density  equal  to  water  the  repulsion 
should  balance  the  gravitational  effect2  when  the  diameter  is  1.5/u 
and  when  it  is  70/xju.  For  particles  of  greater  density  a  balance 
would  be  reached  at  a  still  greater  degree  of  dispersity. 

In  the  very  clear  air  of  Egypt  the  contrast  between  lights  and 
shadows  is  very  marked  because  the  shadows  are  illuminated  but 
slightly  by  diffracted  light  and  are  therefore  darker  than  in 
countries  where  there  is  more  dust  or  moisture.  If  one  is  to  get 
any  detail  in  the  shadows,  one  must  expose  a  negative  about  twice 
as  long3  as  one  would  in  northern  latitudes  under  equally  favor- 
able conditions.  Luckiesh4  points  out  that  a  slight  haze  increases 
the  brightness  of  the  sky  very  much. 

Ait  ken5  has  shown  that  water  vapor  in  itself  does  not  produce 
any  effect  of  haze,  that  haze  is  due  entirely  to  dust,  and  that  the 

1  ALEXANDER:  Colloid  Chemistry,  37  (1919). 

2  Encyclopedia  Britannica,  llth  Ed.,  6,  761  (1910). 

3  BECK:  Jahrbuch  der  Photographic,  18,  159  (1904);  Phot.  Correspondenz, 
1989,  45. 

4  Jour.  Franklin  Inst.,  187,  295  (1919). 

5  Encyclopedia  Britannica,  llth  Ed.,  8,  715  (1910). 


298  APPLIED  COLLOID  CHEMISTRY 

density  of  the  haze  increases  with  the  number  of  the  dust  parti- 
cles in  the  air  and  also  with  the  relative  humidity.  In  Table 
LIII  are  given  Aitken's  results  as  to  the  number  of  particles  of 
atmospheric  dust  in  a  column  of  air  having  a  cross-section  of 
one  square  centimeter  required  to  produce  a  complete  haze, 
that  is,  to  make  a  distant  object  invisible.  The  figures  are  of 
course  not  dependent  on  the  length  of  the  column. 

TABLE  LIII. — PRODUCTION  OF  COMPLETE  HAZE 


Wet-bulb  depressions 

Number  of  particles  to  produce  a  com- 
plete haze 

2°  to    4° 
4°  to    7° 
7°  to  10° 

12,500,000,000 
17,100,000,000 
22,600,000,000 

The  whole  effect  of  light  on  fog  and  haze  may  change  in  a  re- 
markable way  if  one  varies  the  ratio  between  polarized  and  unpol- 
arized  light  as  has  been  brought  out  clearly  by  Tyndall.1  The 
light  of  the  sky  is  polarized  and  may  be  quenched  in  great  part 
by  a  Nicol  prism  while  the  light  from  a  cloud  is  not  polarized 
and  therefore  cannot  be  extinguished.  A  cloud  may  appear  to 
the  naked  eye  dark  against  a  bright  sky  and  yet  appear  as  a  white 
cloud  on  a  dark  ground  if  the  light  from  the  sky  is  quenched  by 
means  of  a  Nicol  prism.  In  the  same  way  the  effect  of  haze  may 
be  overcome  and  a  mountain  may  be  seen  clearly  when  using  a 
Nicol  prism  even  though  almost  invisible  to  the  naked  eye. 
The  elimination  of  polarized  light  in  this  way  was  made  use  of 
during  the  war  to  detect  camouflaged  ships  and  periscopes. 
Still  better  results  were  obtained  with  a  Savart  plate,  the  lines 
disappearing  where  the  ship  was. 

With  ice  crystals  in  the  air,  we  sometimes  get  halos  and  mock 
suns.2  The  reflection  and  refraction  of  the  sunlight  by  small  ice 
crystals  in  the  air  give  rise  to  a  very  complicated  series  of  phe- 
nomena, which  unlike  the  rainbow,  can  be  seen  at  all  altitudes  of 
the  sun.  The  ice  crystals  are  all  hexagonal,  either  flat  plates  or 

1  Fragments  of  Science:  The  Sky  of  the  Alps. 

2  WOOD:  Physical  Optics,  347  (1911);  MOORE:  Descriptive  Meteorology, 
250  (1910). 


FOG  AND  SMOKE  299 

needles  (spiculse).  In  still  air  these  crystals  will  fall  slowly 
with  the  principal  axis  vertical  in  the  case  of  the  flat  plates  or  with 
the  principal  axis  horizontal  in  the  case  of  spiculse,  because 
bodies  moving  through  a  resisting  medium  set  themselves  in  the 
position  of  greatest  resistance.  It  is  possible  to  show  that  the 
observer,  looking  through  a  filmy  cloud  of  such  crystals,  would 
see  in  one  part  of  the  sky  a  halo,  in  another  part  an  arc  of  light, 
and  in  other  directions  bright  spots  like  the  sun,  all  of  them 
arranged  symmetrically  with  regard  to  the  sun  and  the  observer's 
zenith.  This  resulting  complex  arrangement  of  circles  and  spots 
is  known  as  a  complete  halo.  It  is  rare  that  all  possible  com- 
bination are  seen  at  any  one  time,  but  on  several  occasions  solar 
halos  have  been  recorded  that  corresponded  very  nearly  to  this 
ideal. 

The  removal  of  dust  from  the  air  is  an  important  matter  in 
some  industries.  In  photographic  work  dust  particles  in  the 
film  mean  clear  spots  on  the  negative  and  black  ones  on  the 
positive.  In  the  manufacture  of  gas  mantles,  contamination 
by  dust  may  cause  pinholes  in  the  product.  Dust  and  soot  in 
the  air  may  be  a  very  serious  matter  in  the  ventilating  of  elec- 
trical generators  because  65,000  cubic  feet  of  air  may  pass  through 
a  moderately  large  generator  per  minute  or  about  94,000,000 
cubic  feet  per  day.  There  are  a  number  of  firms  which  make 
apparatus  for  washing  and  cooling  air.  All  of  them  remove  the 
dust  by  spraying  drops  of  water  through  the  air  instead  of  bub- 
bling air  through  the  water,  this  latter  method  not  being  effective.1 
Everybody  knows  how  effectively  a  rain  clears  the  air  and  it  is 
this  principle  which  is  employed  commercially. 

The  question  of  flames  is  one  of  colloid  chemistry.  A  flame  is 
a  reacting  gas  and  the  luminosity  is  due  either  to  incandescent 
solid  particles  or  to  chemical  reactions.  The  luminosity  of  ordi- 
nary flames  is  due  chiefly  to  incandescent  particles  of  carbon, 
the  presence  of  which  can  be  shown  by  pressing  a  cooled  dish 
down  upon  the  flame,  when  a  precipitate  of  carbon  is  formed. 
In  the  case  of  flames  colored  by  salts,  the  luminescence  is  due 
chiefly  to  chemical  reactions.2  In  the  green  cupric  chloride 

1  See  however  HURTER:  Jour.  Soc.  Chem.  Ind.,  4,  639  (1885);  6,  707 
(1887);  12,  227  (1893). 

2  WEISER:  Jour.  Phys.  Chem.,  18,  281,  762  (1914);  19,  310  (1915). 


300  APPLIED  COLLOID  CHEMISTRY 

flame,  for  instance,  the  cupric  chloride  dissociates  temporarily 
into  copper  and  chlorine,  the  light  being  due  to  the  recombina- 
tion. The  presence  of  free  copper  in  the  flame  can  be  shown  by 
placing  in  it  a  porcelain  tube  through  which  cold  water  flows. 
Metallic  copper  is  precipitated  as  a  metallic  mirror  on  the  cold 
portion  of  the  tube.  Nearly  all  the  metals  can  be  precipitated 
similarly  from  flames  containing  their  salts,  though  in  some  cases 
the  temperature  of  the  oxyhydrogen  flame  is  necessary  for  a 
satisfactory  dissociation. 

If  the  smoke  particles  are  concentrated  enough  to  form  a  cloud 
which  moves  along,  they  will  tend  to  carry  with  them  the  gases 
inside  the  cloud  because  the  mingling  with  the  air  takes  place 
relatively  slowly  by  diffusion.  At  the  eruption  of  Mont  Pelee 
when  St.  Pierre  was  destroyed,  the  descending  cloud  carried 
down  with  it  large  amounts  of  heated  steam.  The  mean  density 
of  the  water  vapor  and  the  solid  particles  was  greater  than  that  of 
air  and  the  cloud  sank  as  a  whole.  In  the  late  war  the  French 
always  mixed  some  form  of  smoke  with  their  gas  clouds.  That 
had  several  advantages.  It  enabled  the  observers  to  see  what 
the  gunners  were  doing,  it  increased  the  apparent  density  of  the 
cloud,  and  it  caused  the  cloud  to  hold  together  better  than  it 
would  have  otherwise.  It  is  only  fair  to  say,  however,  that  there 
are  no  satisfactory  data  to  show  how  real  these  last  two  advan- 
tages are;  but  one  would  expect  them  to  be  of  less  importance  in 
gas  warfare  where  the  amount  of  smoke  is  small  than  in  the  case 
of  a  volcanic  eruption  where  the  total  amount  of  solid  matter 
in  the  cloud  is  relatively  high.  On  the  other  hand  a  smoke 
cloud  may  hold  together  pretty  well  for  a  mile  or  two,  which  it 
could  hardly  do  if  the  air  in  the  cloud  did  not  move  with  it  to  a 
great  extent. 

Garcia  de  la  Cruz1  found  that  a  hydrogen-filled  balloon  could 
be  adjusted  so  that  it  would  sink  in  pure  air  and  would  rise  in 
air  containing  any  kind  of  smoke. 

PRECIPITATION  OF  SOLID 

There  seems  to  have  been  no  systematic  study  of  the  effect  of 
conditions  on  the  form  in  which  a  solid  precipitates  from  a  gas 

1  SPRING:  Mem.  Soc.  beige  Geologic,  17,  30  (1903). 


FOO  AND  SMOKE  301 

or  vapor,  although  there  are  a  good  many  scattered  observations. 
With  carbon  the  matter  is  important,  in  its  bearing  on  the  pro- 
duction of  lamp-black,  charcoal,  coke  and  metallized  filaments. 
Monier1  states  that  carbon  made  from  pure  sugar  is  very  hard 
and  will  scratch  topaz  though  not  corundum.  Schiitzenberger2 
reports  that  when  cyanogen  is  passed  over  carbon  soaked  with 
cryolite  and  heated  to  a  cherry-red  heat,  it  decomposes  into 
nitrogen  and  filamentous  carbon.  Constant  and  Pe*labon3  have 
also  made  observations  on  filamentous  carbon.  Love4  reports 
on  the  carbon  deposits  obtained  in  gas  burners.  Luzi5  obtained 
a  silvery  graphite  by  decomposition  of  hydrocarbons  at  the 
surface  of  white-hot  porcelain.  Bone  and  Coward6  believe  that 
retort  carbon  is  obtained  essentially  from  methane  because  the 
carbon  produced  by  cracking  methane  differs  materially  from 
that  formed  from  the  other  hydrocarbons.  In  the  early  days 
of  the  war  it  was  believed  that  a  good  adsorbent  charcoal  was  to 
be  obtained  only  by  the  decomposition  of  certain  hydrocarbons; 
but  that  view  did  not  last  long.  The  work  of  Chaney  of  the 
National  Carbon  Company  has  thrown  some  light  on  portions 
of  this  field.  Carbon  obtained  at  low  temperatures  is  adsorbent. 
If  formed  above  600°  it  is  what  Chaney  calls  gas-treated  and  is  not 
adsorbent.  The  higher  the  temperature,  the  more  the  deposit  is 
like  graphite.  Bone  and  Coward  are  wrong  in  thinking  that 
carbon  from  methane  is  necessarily  characteristic.  The  differ- 
ence is  due  to  the  fact  that  a  higher  temperature  is  necessary. 
While  it  is  of  course  possible  that  carbon  from  different  hydro- 
carbons may  be  different  apart  from  temperature  conditions, 
there  is  nothing  at  present  to  show  this. 

Becker7  claims  that  when  vapors  condense  to  solids,  they 
pass  direct  to  that  form  without  any  intermediate  formation 
of  liquid.  Of  course  this  cannot  always  be  true  and  can  only 
apply  to  the  particular  cases  and  conditions  under  consideration 
by  Becker. 

1  Jour.  Chem.  Soc.,  27,  674  (1874). 

2  Ibid.,  60,265  (1891). 

3  Comptes  rendus,  137,  706  (1903). 

4  Jour.  Soc.  Chem.  Ind.,  12,  433  (1893). 

5  Jour.  Chem.  Soc.,  62,  565  (1892). 

6  Ibid.,  93,  1211  (1908). 

7  Zeit.  phys.  Chem.,  78,  39  (1912). 


302  APPLIED  COLLOID  CHEMISTRY 

Dewar1  states  that  if  a  glass  rod  is  cooled  to  the  temperature 
of  liquid  air  and  is  then  brought  into  the  air  of  the  room,  moisture 
will  condense  on  it  as  a  sheet  of  ice.  If  the  glass  rod  is  electrified 
by  contact  with  a  piece  of  silk,  the  ice  forms  as  a  forest  of  crystals 
and  not  as  a  sheet.  The  reason  for  this  seems  to  be  that  the  ice 
particles  are  themselves  electrified  and  consequently  precipitate 
as  far  from  each  other  as  possible. 

Platinum  mirrors  were  obtained  by  Wright2  by  electrical  dis- 
integration of  platinum  wires  and  by  Edison3  by  volatilization  of 
electrically  heated  platinum  in  hydrogen.  Cottrell4  modified  the 
method  of  electrical  disintegration  so  as  to  obtain  platinum  black. 

Any  study  of  the  form  in  which  solids  precipitate  should  extend 
to  precipitation  from  solution,  etc.,  and  should  include  a  study 
of  the  theory  of  the  formation  of  mirrors,5  of  the  way  in  which 
silver  precipitates,6  of  mossy  copper,7  of  filamentous  potassium 
chloride,8  of  silver  chloride  growths  with  silver,  sodium  chloride, 
and  gelatine,9  etc. 

One  particular  case  of  precipitation  is  so  puzzling  as  to  call  for 
special  mention.  If  hydrogen  is  passed  into  a  saturated  aqueous 
solution  of  silver  oxide  containing  an  excess  of  the  solid  salt, 
silver  precipitates  to  some  extent  as  crystals  and  in  part  as  col- 
loidal silver. 10  The  form  in  which  the  metal  comes  down  depends 
on  the  nature  of  the  containing  vessel.  In  a  platinum  vessel  no 
hydrosol  is  formed  and  all  the  silver  precipitates  in  a  crystalline 
form  on  the  walls  of  the  vessel.  In  vessels  of  quartz  and  of 
ordinary  glass,  the  colloidal  silver  is  yellowish-brown  by  trans- 
mitted light,  while  it  comes  down  red  to  blue  in  a  flask  made  of 
Jena  glass.  The  ratio  of  silver  hydrosol  to  ordinary  silver  is 

Chem.  News,  97,  5  (1908). 
Am.  Jour.  Sci.  (3)  14,  169  (1877). 
Chem.  News,  40,  152  (1879). 
Zeit.  phys.  Chem.,  42,  385  (1903). 

Cf.  WADSWORTH:  Zeit.  Instrumentenkunde,  15,  22  (1895);  NEOGI:  Zeit. 
anorg.  Chem.,  59,  213  (1906);  CHATTAWAY:  Proc.  Roy.  Soc.,  80A,  88  (1908). 
6Cf.  KOHLSCHUTTER:  Zeit.  Elektrochemie,  14,  49  (1908);  18,  373,  419 
(1912);  Liebig's  Ann.,  387,  86;  309,  340  (1912);  398,  47  (1913);  PHILLIPS: 
Jour.  Chem.  Soc.,  72  II,  32  (1897). 

'HUTCHINGS:  Jour.  Chem.  Soc.,  32,  113  (1877). 
8  WARINGTON:  Jour.  Chem.  Soc.,  8,  30  (1856). 
OLUPPO-CRAMER:  Zeit.  Kolloidchemie,  9,  116  (1911). 
10  KOHLSCHUTTER:  Zeit.  Elektrochemie,  14,  49  (1908). 


FOG  AND  SMOKE  303 

greater  in  the  Jena  glass  vessel  than  in  the  other  two.  At  first 
sight  one  would  expect  this  difference  in  behavior  to  be  due  to 
differences  in  material  dissolved  from  the  walls;  but  this  is  not 
the  case.  Kohlschtitter  allowed  water  to  stand  in  an  ordinary 
glass  flask  for  a  while  and  then  poured  it  into  the  Jena  glass 
flask.  The  reduction  product  was  red.  When  water  which  had 
stood  in  a  Jena  glass  flask  was  poured  into  an  ordinary  flask  or 
into  a  quartz  one,  the  silver  came  down  yellowish-brown.  The 
phenomenon  is  therefore  connected  with  the  presence  and  nature 
of  the  solid.  The  most  plausible  explanation  is,  as  suggested  by 
Kohlschlitter,  that  the  reaction  concentrations  are  highest  at  the 
surface  of  the  platinum  and  lowest  at  the  surface  of  quartz  and 
the  ordinary  glass,  so  that  the  silver  comes  down  coarsest  and 
most  crystalline  in  platinum  vessels  and  finest  in  quartz.  This 
could  be  checked  experimentally  by  determining  the  adsorbing 
power  of  platinum,  quartz,  and  Jena  glass  for  silver  oxide  and  for 
colloidal  silver.  It  is  probable  that  the  adsorption  is  greatest 
with  platinum  and  least  with  quartz.  This  behavior  of  the  silver 
may  be  connected  with  the  fact  that  it  is  easier  to  get  a  yellow 
stain1  of  silver  on  a  potash-lime  glass  than  on  the  hard  glasses. 
Bredig2  made  use  of  the  arc  under  water  between  wire  termi- 
nals to  give  him  colloidal  solutions  of  platinum  and  other  metals. 
Schoop3  has  developed  a  process  for  plating  metals  on  all  sorts 
of  materials  by  blowing  compressed  air  at  about  five  atmospheres 
pressure  through  a  pointed  nozzle  into  an  arc  playing  between 
two  wire  terminals.  Smoo  h  well-adhering  deposits  of  sprayed 
metal  are  obtained  when  aluminum  or  copper  is  sprayed  on  glass 
in  this  way,  and  the  metal  adheres  so  tenaciously  that  it  is  im- 
possible to  remove  the  film  without  destroying  the  glass  surface. 
It  is  claimed  that  a  microscopic  examination  showed  that  the 
surface  of  the  glass  melts  and  that  the  metal  is  practically  fused 
into  the  glass.  This  will  happen  only  when  the  object  to  be 
coated  is  placed  very  close  to  the  arc.  By  working  at  a  longer 
range  it  is  possible  to  coat  combustible  materials  like  paper  and 
celluloid.  It  is  possible  to  obtain  deposits  of  brass  by  making  one 
pole  of  copper  and  the  other  of  zinc. 

1  ROSENIIAIN:  Glass  Manufacture,  185  (1908). 

2Zeit.  Elektrochemie,  4,  514  (1898);  Zeit.  phys.  Chem.,  31,  258  (1899). 

3  Chem.  Abs.,  13,  2640  (1919). 


CHAPTER  XII 
GASES  AND  SOLIDS  IN  SOLIDS 

GASES  IN  SOLIDS 

Reusch1  points  out  that  the  volcanic  ash  erupted  from  a  vol- 
cano often  consists  of  an  obsidian  glass  filled  with  innumerable 
gas  bubbles  and  that  consequently  large  pieces  will  float  on  water. 
If  the  expansion  of  the  gases  is  too  great  a  glass  foam  may  be 
obtained  which  may  break  down  into  fragments  of  glass. 
Hannay2  exposed  glass  at  200°  to  oxygen  and  to  carbon  dioxide 
at  200  atmospheres  pressure,  and  allowed  the  glass  to  cool  under 
pressure.  So  much  gas  was  taken  up  that  when  the  glass  was 
heated  quickly  it  passed  into  a  foam.  It  would  probably  have 
been  possible  to  have  regulated  the  pressure  and  the  rate  of  heat- 
ing so  as  to  have  obtained  small  but  visible  bubbles  in  the  glass. 
In  fact  this  has  been  done  by  others  when  making  artificial 
pumice.3  As  has  been  pointed  out  previously,  it  is  very  probable 
that  the  blue  color  of  many  feathers  is  due  to  the  presence  of 
minute  bubbles  in  the  horny  part  of  the  feather.4  When  these 
are  seen  against  a  dark  back-ground  we  get  a  blue  analogous  to 
the  blue  of  the  sky,  due  to  suspended  particles  in  a  homogeneous 
medium.  In  the  case  of  the  sky  the  particles  are  solids  or  liquids 
suspended  in  a  gas.  In  the  case  of  the  feathers,  we  have  air 
bubbles  in  a  solid  medium. 

The  floating  soaps  contain  injected  air  bubbles  and  are  there- 
fore suspensions  of  gas  in  a  solid.  Bread  may  also  be  included 
under  this  head.  On  the  other  hand  a  porous  plate  is  really  an 
interlacing  system  because  the  gas  phase  is,  or  should  be,  con- 
tinuous. Porous  silica  gels  occur  as  opals,  as  hydrophane,  and 
tabaschir,  this  last  being  found  in  bamboos.  In  so  far  as  they 

1  Jour.  Chem.  Soc.,  46,  415  (1884). 

2  Proc.  Roy.  Soc.,  32,  407  (1881). 

3  Jour.  Franklin  Inst.,  174,  344  (1912). 

4  HAECKER:  Zool.  Jahrbucher  Abt.  syst.  Geog.  Biol.  Tiere,  16,  267  (1920). 

304 


GASES  AND  SOLIDS  IN  SOLIDS  305 

are  porous  they  are  also  interlacing  systems.     Meerschaum  is  a 
silicate  mineral  coming  under  the  same  head. 

A  special  case  of  gases  and  solids  is  a  laminated  system. 
Strictly  speaking,  the  gas  phase  is  continuous;  but  one  cannot 
treat  it  as  a  case  of  a  solid  dispersed  in  a  gas.  Since  the  air  film 
is  very  thin,  it  seems  better  to  consider  the  theory  of  lamination 
here  rather  than  elsewhere.  This  has  been  discussed  in  detail 
by  Tyndall1  who  showed  that  a  laminated  structure  is  produced 
if  an  apparently  homogeneous  mass  is  subjected  to  pressure 
under  such  conditions  that  it  can  yield  at  right  angles  to  the 
pressure.2  He  illustrated  this  by  experiments  on  a  mass  of  pure 
white  wax.  When  chilled  and  compressed,  it  could  then  be  split 
into  laminae  of  extraordinary  thinness.  The  same  phenomenon 
occurs  on  a  large  scale  in  nature  with  many  slates.  Tyndall 
points  out  that  we  do  the  same  thing  in  the  kitchen  when  making 
puff-paste.  "The  cleavage  of  our  hills  is  accidental  cleavage, 
but  this  is  cleavage  with  intention.  The  volition  of  the  pastry 
cook  has  entered  into  its  formation.  It  has  been  his  aim  to 
preserve  a  series  of  surfaces  of  structural  weakness,  along  which 
the  dough  divides  into  layers.  Puff-paste  in  preparation  must 
not  be  handled  too  much;  it  ought,  moreover,  to  be  rolled  on  a 
cold  slab  to  prevent  the  butter  from  melting  and  diffusing  itself, 
thus  rendering  the  paste  more  homogeneous  and  less  liable  to 
split.  Puff-paste  is,  then,  simply  an  exaggerated  case  of  slaty 
cleavage." 

SOLIDS  IN  SOLIDS 

Adsorption  of  a  solid  by  a  solid  is  not  properly  to  be  considered 
as  a  case  of  a  solid  dispersed  in  a  solid  because  the  adsorption  is  a 
surface  phenomenon.  Consequently  such  things  as  the  pigment 
lakes  are  excluded,  though  it  is  possible  that  after  ignition  they 
change  their  type.  On  the  other  hand  the  ultra-microscope 
shows  the  presence  of  a  second  phase  in  the  case  of  ruby  glass 
colored  by  gold.  The  ruby  glass  has  some  very  interesting  prop- 
erties.3 Fused  glass  containing  a  small  amount  of  gold  is  color- 
less and  remains  colorless  if  cooled  quickly.  If  cooled  slowly  or 

1  Fragments  of  Science:  On  Crystalline  and  Slaty  Cleavage. 

2  See,  however,  BECKER:  Bull.  U.  S.  Geol.  Survey  No.  241  (1904);  LEITH; 
Ibid.  No.  239  (1905). 

3  ZSIGMONDY:  Colloids  and  the  Ultra-microscope,  165  (1909). 

20 


306  APPLIED  COLLOID  CHEMISTRY 

if  re-heated  to  a  temperature  well  below  the  melting-point,  it 
becomes  red  owing  to  the  separation  of  particles  of  gold  of  the 
size  to  transmit  red  light.  If  held  too  long  at  a  high  temperature 
the  red  color  changes  to  a  violet  and  then  to  a  blue.  The  changes 
are  exactly  those  which  we  get  in  aqueous  solutions  and  for  the 
same  reason,  the  increase  in  the  size  of  the  particles.  It  is  not 
quite  clear,  however,  why  the  chilled  glass  should  be  colorless. 
This  cannot  be  due  to  the  formation  of  a  gold  salt  because  the 
same  phenomenon  is  observed  in  a  reducing  atmosphere.  It 
must  either  be  that  the  gold  is  in  true  solution  in  the  glass  or 
that  it  has  been  so  peptized  by  the  glass  that  the  colloidal  parti- 
cles are  too  small  to  have  any  effect  upon  the  beam  of  light. 
Zsigmondy  believes  that  both  things  occur  though  he  assumes 
without  any  apparently  adequate  reason  that  most  of  the  gold  is 
in  true  solution.  It  would  probably  be  easy  to  settle  this  defi- 
nitely if  experiments  were  made  with  a  glass  richer  in  gold.  With 
stannic  oxide  in  the  glass  there  is  less  tendency  for  he  gold  to 
turn  blue  on  prolonged  heating  and  it  is  therefore  easier  to  match 
colors.1  The  tinctorial  power  of  the  gold  is  very  high,  0.01  per- 
cent giving  a  pink  color  and  0.1  percent  a  red.  The  number  of 
colloidal  particles  is  enormous,  billions  per  cubic  millimeter 
according  to  Zsigmondy. 

The  cheaper  form  of  ruby  glass  owes  its  color  to  colloidal 
copper2  and  the  red  in  some  glazes  is  due  to  copper  in  some  form. 
The  Chinese  ox-blood  is  believed  by  Lauth  and  Dutailly  to  be 
metallic  copper  adsorbed  by  tin  oxide.  On  rapid  cooling  a  color- 
less glaze  is  produced,  which  turns  red  on  slow  heating.  Seger3 
does  not  agree  with  the  view  that  the  red  color  is  due  to  a  segrega- 
tion of  metallic  copper.  His  experiments  showed  that  alternately 
reducing  and  oxidizing  conditions  of  the  fire  gases  are  necessary  to 
produce  the  red  color.  He  considers  that  the  color  is  due  to 
cuprous  oxide  or  cuprous  silicate.  This  opinion  is  also  shared 
by  Stokes4  who  believes  that  the  color  transmitted  by  colloidal 
copper  is  blue,  whereas  this  is  the  color  of  the  coarser  colloidal 
copper. 

Bellamy:  Jour.  Am.  Ceramic  Soc.,  2,  313  (1919). 

2  WILLIAMS:  Trans.  Am.  Ceramic  Soc.,  16,  284  (1914). 

3  Collected  Writings,  2,  734  (1902). 

4  Mathematical  and  Physical  Papers,  4,  245  (1904). 


GASES  AND  SOLIDS  IN  SOLIDS  307 

Rosenhain1  seems  to  be  quite  uncertain  whether  the  color  of  a 
copper  ruby  glass  is  due  to  metallic  copper  or  to  cuprous  oxide. 
In  Aventurine  glass,  the  second  phase  comes  out  in  spangles, 
presumably  of  metallic  copper.  Zulkowski2  believes  that  copper 
ruby  glass  owes  its  color  to  cuprous  oxide  but  the  general  opinion 
is  against  him,  though  the  evidence  is  not  very  conclusive.3  The 
truth  of  the  matter  probably  is  that  we  may  have  metallic  cop- 
per in  one  glass  and  cuprous  oxide  in  another.  At  present  there 
is  no  method  of  differentiating  these  two  cases.  On  the  other 
hand,  it  ought  to  be  a  simple  matter  to  tell  whether  one  were 
dealing  with  cuprous  oxide  or  cuprous  silicate  because  a  cuprous 
silicate  glass  should  be  optically  empty  whereas  a  cuprous  oxide 
glass  should  scatter  light.  It  is  not  easy  to  see  why  a  cuprous 
silicate  glass  should  become  colorless  when  fused.  Berry4 
describes  a  red  slag  from  a  copper  refinery  which  contained  some 
metallic  copper,  though  not  much.  When  treated  with  acids 
about  27  percent  Cu20  dissolved,  leaving  a  pale  yellow  slag 
containing  8.6  percent  Cu20,  11.7  percent  FeO,  and  49.3  per- 
cent SiO2,  the  percentages  referring  to  the  original  composition. 
Berry  considers  that  the  pale  yellow  is  the  resultant  of  green 
due  to  the  iron  and  red  due  to  the  silver. 

Silver  yields  a  beautiful  yellow  color5  when  applied  to  glass  as 
a  surface  stain.  This  is  the  color  of  colloidal  silver  solutions 
having  the  finest  particles.  The  best  yellow  stain  is  obtained 
with  potash-lime  glasses  as  the  silver  tends  to  come  down  metallic 
on  the  hard  glasses.  In  the  case  of  enamels,  silver  is  added  to 
the  mass  of  the  glaze6  and  not  as  a  surface  stain.  Addition  of 
silver  carbonate  together  with  bismuth  oxide  gives  rise  to  an 
intense  blue.  One  function  of  the  bismuth  oxide  is  to  hold  the 
silver  to  the  body;  but  it  must  also  cause  a  partial  agglomeration 
of  the  silver  because  the  lustres  are  blue  to  green  instead  of  yellow 
to  brown.  A  yellow  stain  of  silver  on  a  cobalt  blue  enamel  gives 
a  green.  This  is  a  ceramic  analogue  of  the  tree  frog. 

1  Glass  Manufacture,  182,  184  (1908). 

2  Jour.  Soc.  Chem.  Ind.,  16,  441  (1897). 

3  THORPE:  Dictionary  of  Applied  Chemistry,  2,  721  (1918). 

4  Am.  Chem.  Jour.,  8,  429  (1887). 

6 ROSENHAIN:  Glass  Manufacture,  185  (1908). 
6FBANCHETI  Ann.  Chim.  Phys.  (8)  9,  37  (1906). 


308  APPLIED  COLLOID  CHEMISTRY 

Platinum  and  iridium  oxide  can  be  used  to  give  grays,  especi- 
ally in  enamels,  but  they  are  expensive.  Glass  can  be  colored 
yellow  by  suspended  carbon,  the  color  of  thin  films  of  so-called 
carbon  being  yellow.  Carbon  cannot  be  used  in  lead  glasses 
because  of  its  reducing  action. 

Glasses  containing  no  heavy  metals  are  colored  yellow  to 
yellowish  brown  by  sulphur;  but  this  is  apparently  due  to  forma- 
tion of  polysulphides  because  these  glasses  are  optically  empty.1 
In  calcium  glasses  addition  of  sulphur  gives  a  blue  color,  ap- 
parently due  to  the  presence  of  colloidal  sulphur.  We  do  not 
know  at  all  why  colloidal  sulphur  should  ever  be  blue  but  ap- 
parently it  may  be.  Wolfgang  Ostwald2  states  that  blue  and 
green  colloidal  solutions  of  sulphur  can  be  obtained  by  introducing 
sulphur  into  molten  sodium  chloride,  into  a  borax  bead,  into 
liquid  ammonia  or  into  hot  organic  liquids  like  glycerine.  He 
considers  that  this  blue  colloidal  sulphur  is  the  cause  of  the  blue 
color  of  ultramarine.3 

In  strongly  alkaline  glasses  selenium  gives  a  chestnut  brown 
color  and  the  glasses  are  then  optically  empty.4  In  moderately 
alkaline  glasses  the  color  is  red  to  pink  and  submicrons  can  be 
seen.  Potash  glasses  give  a  purer  red  than  soda  glasses.  If  the 
red  glasses  are  heated  to  620°,  the  color  becomes  a  pale  brownish 
yellow.  Sodium  borate  glasses  containing  selenium  are  often 
yellow,  but  turn  red  when  annealed.  Colloidal  selenium  solu- 
tions can  easily  be  prepared  which  are  red  by  transmitted  light,5 
almost  exactly  the  color  of  arterial  blood. 

In  low  concentrations  tellurium  colors  glass  coral  to  purple-red 
and  in  higher  concentrations  a  steel  blue.  It  is  possible  to  make 
a  blue  colloidal  solution  of  tellurium  by  reducing  a  dilute  boiling 
mixture  of  sodium  tellurate  and  sodium  protalbinate  with  aqueous 
hydroxylamine  hydrochloride.  It  would  probably  be  easy  to 
make  a  coral-red  solution  of  colloidal  tellurium  though  this 
appears  never  to  have  been  done.6 

IFENABOLI:  Zeit.  Kolloidchemie,  16,  53  (1915). 

2  Theoretical  and  Applied  Colloid  Chemistry,  187  (1917). 

3  See  also  MORGAN:  Jour.  Oil  and  Colour  Chemists'  Ass.,  2,  120  (1919). 
4FENAEOLi:  Zeit.  kolloidchemie,  16,  53  (1915). 

6  SCHULZE:  Jour,  prakt.  Chem.  (2),  32,  390  (1885). 
6BEBZELius:  Ann.  Phys.  (2)  32,  1  (1834). 


GASES  AND  SOLIDS  IN  SOLIDS  309 

Enamels'  are  "  vitreous  compositions  capable  of  adhering  by 
fusion  to  the  surface  of  metal  or  of  pottery.  The  base  of  the 
enamel  is  generally  a  colorless  glass  in  which  are  suspended  parti- 
cles of  an  opaque  metallic  oxide  or  salt,  generally  stannic  oxide. 
Up  to  about  900°  tin  dioxide  exists  finely  suspended  in  the 
enamel,  but  ta  a  higher  temperature  combination  or  solution  may 
occur  and  opacity  is  diminished.  Titanic  oxide  has  been  used  in 
place  of  stannic  oxide,  and  antimony  compounds  have  been 
employed.  Opacity  may  also  be  obtained  by  the  use  of  arsenious 
oxide,  calcium  phosphate,  cryolite,  or  fluorspar.  A  preparation 
known  as  artificial  cryolite  sometimes  replaces  the  mineral. 
When  bone-ash  is  used,  a  little  nitre  may  be  added  to  remove  the 
color." 

The  opacity  produced  by  these  added  white  compounds  is  due 
to  the  difference  in  refractive  index.  Natural  tin  oxide,  for 
instance,  has  a  much  higher  refractive  index  for  yellow  light  than 
glass.  While  titania  has  been  used  as  a  substitute  for  stannic 
oxide,  it  is  not  a  very  good  one  because  it  gives  a  yellow  tint. 
Haber2  states  that  six  million  kilos  of  stannic  acid  a  year  were 
used  for  enamelling  iron  and  that  this  amount  was  halved  when 
people  stopped  mixing  and  melting  the  oxide  with  the  other  con- 
stituents of  the  glaze  and  merely  added  it  to  the  enamel  just 
before  baking.  In  the  old  process  a  great  deal  of  the  stannic 
oxide  was  actually  dissolved  in  the  glass  and  consequently  did  not 
increase  the  opacity.  Impure  zirconia  is  now  being  used  as  a 
substitute  for  tin  oxide  in  producing  opacity.  If  the  zirconia 
goes  into  solution  as  oxide  or  as  silicate,  it  does  not  cause 
opacity.3  Of  five  crystalline  titania  glazes  which  were  examined,4 
four  contained  rutile  and  one  probably  asgenite.  Almost  all  the 
titania  had  crystallized.  In  a  crystalline  zinc  glaze  the  crystals 
resembled  willemite.  An  opaque  yellow  lead  glass  is  obtained 
with  antimony  oxide,  the  second  phase  being  apparently  lead 
antimoniate,  sometimes  called  Naples  Yellow.  The  black  lustre 
of  the  Greek  and  Roman  pottery  has  been  reproduced5  in  an 

1  THORPE:  Dictionary  of  Applied  Chemistry,  2,  333  (1918). 

2  Jour.  Soc.  Chem.  Ind.,  33,  49  (1914). 

3  GRUNWALD:  Jour.  Soc.  Chem.  Ind.,  30,  210  (1911). 
4ENDELL:  Jour,  Soc.  Chem.  Ind.,  30,  210,  489  (1911). 
6  FRANCHET:  Comptes  rendus,  162,  1097  (1911). 


310  APPLIED  COLLOID  CHEMISTRY 

oxidizing  fire  at  850°  by  fluxing  55  parts  of  quartz  sand  with  45 
of  sodium  carbonate  and  grinding  the  frit  with  an  equal  weight  of 
magnetite.  Artificial  ferroso-ferric  oxide  does  not  yield  the 
black  color  which  is  probably  enhanced  by  traces  of  manganese 
in  the  natural  ore.  The  opacity  of  the  enamel  is  caused  by  the 
presence  of  more  stain  than  will  dissolve  in  the  flux.  Franchet 
states  that  the  black  enamel  was  known  to  the  Egyptians  and 
consequently  was  not  discovered  by  the  Greeks.  Foster1  con- 
siders that  ferrous  iron,  presumably  as  ferrous  silicate,  is  the 
cause  of  the  black  color;  but  Franchet 's  experiments  seem  pretty 
conclusive. 

Many  of  the  natural  obsidians2  which  appear  black  consist  of 
a  colorless  matrix  swarming  with  specks  of  magnetite.  If  there 
is  complete  oxidation  to  ferric  oxide  the  color  is  red.  Carnelian 
is  chalcedony  containing  globules  of  hematite.  If  the  amount  of 
dispersed  material  is  sufficient  to  make  the  mass  opaque,  we 
have  jasper,3  which  is  red  with  hematite,  yellow  with  limonite 
and  green  with  saponite  or  chlorite.  Jasper  may  contain  up  to 
twenty  percent  suspended  material. 

Blue  quartz  is  not  uncommon  in  the  Southern  Appalachians 
and  the  color  is  apparently  due  to  the  presence  of  hair-like 
inclusions  of  rutile.4  Since  the  index  of  refraction  of  rutile  is 
very  different  from  that  of  quartz,  this  is  apparently  similar 
to  blue  feathers,  except  that  we  have  here  solid  particles  instead 
of  minute  bubbles.  Iddings5  states  that  the  blue  color  of  the 
opalescent  quartz  from  Llano  County,  Texas,  "is  undoubtedly 
due  to  reflection  of  blue  light  from  the  minute  colorless  prisms 
whose  width  is  a  fraction  of  the  length  of  light  waves." 

Egyptian  blue6  is  crystallized  CaO.CuO.4SiO2.  It  is  formed 
between  800°  and  900°,  above  900°  a  green  glass  being  obtained. 
The  reaction  is  reversible,  for  blue  crystals  are  formed  if  the 
green  glass  is  held  at  850°. 

Glass  can  be  colored  yellow  with  colloidal  cadmium  sulphide 

i  Jour.  Am.  Chem.  Soc.,  32,  1259  (1910). 
2PiRSsoN:  Am.  Jour.  Sci.  (4)  30,  104  (1910). 

3  GOOD  CHILD:  Precious  Stones,  174  (1908). 

4  WATSON  and  BEARD:  Proc.  Nat.  Museum,  53,  561  (1917). 
6  Jour.  Geology,  12,  227  (1904). 

6 LAURIE:  Proc.  Roy.  Soc.,  89A,  418  (1914). 


GASES  AND  SOLIDS  IN  SOLIDS  311 

and  Homer1  has  produced  a  yellow  of  this  sort  in  a  boric  acid 
bead.  It  really  would  be  a  good  thing  if  somebody  were  to  take 
up  the  whole  question  of  colors  in  the  borax  bead  so  as  to  deter- 
mine which  were  due  to  colloids  and  which  to  true  solutions; 
and  also  to  find  out  definitely  what  chemical  compounds  cause 
the  color  in  each  particular  case. 

Rock  salt  has  been  found  having  a  blue  color  which  is  due 
apparently  to  colloidal  particles  of  metallic  sodium.  The  same 
result  can  be  obtained  by  heating  salt  with  sodium  vapor2  or  by 
exposing  salt  to  cathode  rays.  Radium  emanation  colors  sodium 
glasses  blue  to  violet  and  lead  glasses  brown. 

The  action  of  heat,  ultra-violet  light,  and  radium  on  the  colors 
of  gems  is  very  interesting3  and  the  theory  of  it  has  not  been 
worked  out  satisfactorily.  Pale  amethysts  become  darker  when 
exposed  to  radium,  while  ultra-violet  light  has  no  effect.  When 
heated  to  redness  in  hydrogen  or  oxygen,  the  pale  amethysts 
become  colorless  while  they  turn  yellow  if  heated  in  ammonia. 
The  decolorized  amethysts  regain  their  color  when  treated  with 
radium.  Rose  quartz  is  made  colorless  by  ultra-violet  light 
and  blackish-brown  by  radium.  It  is  not  changed  when  heated 
in  ammonia.  Smoky  quartz  loses  its  color  when  heated  and 
radium  brings  it  back,  while  hydrogen  peroxide  tends  to  make 
the  color  yellower.  Colorless  topaz  is  made  yellow  to  orange 
by  radium  and  is  decolorized  when  heated.  Ultra-violet  light 
tends  to  change  the  orange  produced  by  radium  to  lilac.  Kun- 
zite  changes  from  lilac  to  green  under  the  influence  of  radium 
and  is  changed  back  by  ultra-violet  light.  It  becomes  colorless 
when  heated  to  400°;  but  exposure  to  radium  brings  back  the 
blue-green  color.  Corundum  occurs  as  blue,  green,  violet, 
yellow  and  white  sapphires  and  as  ruby.  The  Oriental  sapphire 
is  said  by  Verneuil4  to  be  colored  by  iron  and  titanium  while  the 
clear  sapphire  is  colored  by  iron  only.  Blue  sapphires  are 
changed  to  yellow  or  yellowish-brown  by  radium,  the  blue-green 
sapphires  to  green  and  the  white  sapphires  to  yellow.  Violet 
sapphires  become  pure  red  and  natural  rubies  lose  any  violet 

1  Chem.  News,  29,  66  (1874). 

2SiEDENTOPF:  Verb,  deutsch.  phys.  Ges.,  7,  268  (1905). 
3  DOELTER:  Das  Radium  und  die  Farben  (1910). 
4Comptes  rendus,  161,  1053  (,1910). 


312  APPLIED  COLLOID  CHEMISTRY 

tinge .  Artificial  rubies  and  sapphires  are  no  t changed  by  exposure 
to  radium,  but  their  coloring  matter  is  chromium  or  cobalt. 
Ultra-violet  light  makes  yellow  sapphires  blue  and  violet  ones 
more  violet.  Heating  sapphires  in  air  makes  htem  colorless. 

The  general  result  seems  to  be  in  all  cases  that  heating  makest  he 
gems  more  nearly  colorless1  and  that  the  action  of  radium  and  of 
ultra-violet  light  is  antagonistic.  The  only  possible  explanation 
seems  to  be  that  radium  increases  the  dispersity  of  the  colloidal 
particles  while  ultra-violet  light  decreases  it  or  vice-versa.  We 
know  that  /3  rays  increase  the  agglomeration  of  sulphur  and 
that  they  change  a  selenium  hyrosol  into  crystalline  selenium. 
It  should  be  possible  to  test  this  explanation  on  synthetic  ma- 
terials, using  perhaps  borate  glasses.  For  instance,  radium 
produces  no  change  in  pure  chromic  oxide  but  turns  it  brown 
when  the  chromic  oxide  is  dissolved  in  borax.  Ultra-violet 
light  changes  the  brown  to  yellow  and  when  a  chromium  oxide 
borax  glass  is  heated  in  ammonia  it  becomes  pale.  Alumina  is 
not  changed  by  radium  but  hydrous  aluminum  oxide  sol  is 
turned  blue  by  it.  If  cases  of  this  sort  should  be  studied  care- 
fully it  would  probably  give  us  the  necessary  data  to  straighten 
out  the  question  of  the  colors  of  gems  without  any  difficulty. 
It  is  to  be  noticed  that  copper  oxide  adsorbed  by  alumina  is 
apparently  blue,2  and  it  is  quite  possible  that  magnetite  may  be. 

The  presence  of  surfaces  of  discontinuity  may  give  rise  to 
color  effects  without  any  pigment  being  present.  In  mother-of- 
pearl  we  have  alternate  layers  of  calcium  carbonate  and  of 
organic  matter  called  conchiolin.3  On  the  outside  each  layer 
projects  slightly  beyond  the  edge  of  the  preceding  so  that  the 
outside  of  the  nacreous  portion  shows  innumerable  steps  or  ridges 
running  parallel  to  the  growing  edge  of  the  shell,  thus  producing 
the  effect  of  a  diffraction  grating.  The  color  of  mother-of-pearl 
is  due  to  the  diffraction  of  light  from  this  grating-like  structure 
and  to  the  interference  of  light  by  reflection  from  numerous 
parallel  laminae  of  sensibly  equal  thickness. 

Some  crystals  of  potassium  chlorate  show  brilliant  colors  and 

1  Cf.  HAMLIN:  The  Tourmaline,  87  (1873). 

2  SCHENCK:  Jour.  Phys.  Chem.,  23,  283  (1919). 

3  PFUND:  Jour.  Franklin  Inst.,  183,  453  (1917);  SCHADE:  Kolloidchemische 
Beihefte,  1,  385  (1910). 


GASES  AND  SOLIDS  IN  SOLIDS  313 

this  was  found  by  Stokes  to  be  due  to  the  existence  of  planes  within 
the  crystals  at  which  a  periodic  twinning  had  occurred  and  where 
there  was  probably  a  surface  of  discontinuity.1  The  same  phe- 
nomenon is  shown  by  the  opal  though  it  is  not  crystalline.  Lord 
Rayleigh2  assumes  that  the  reflection  takes  place  at  a  number  of 
thin  laminse  sensibly  equidistant,  the  distance  between  any 
two  being  of  the  order  of  magnitude  of  the  light-wave.  Crookes3 
says :  "  When  a  good  fiery  opal  is  examined  in  daylight,  sunlight  or 
artificial  light,  it  appears  to  emit  vivid  flashes  of  crimson,  green, 
or  blue  light,  according  to  the  angle  at  which  the  incident  light 
falls,  and  the  relative  position  of  the  opal  and  the  observer;  for 
the  direction  of  the  path  of  the  emitted  beam  bears  no  uniform 
proportion  to  the  angle  of  the  incident  light.  Examined  more 
closely,  the  flashes  of  light  are  seen  to  proceed  from  planes  or 
surfaces  of  irregular  dimensions  inside  the  stone,  at  different 
depths  from  the  surface  and  at  all  angles  to  each  other.  Occasion- 
ally a  plane  emitting  light  of  one  color  overlaps  a  plane  emitting 
light  of  another  color,  the  two  colors  becoming  visible  alternately 
upon  slight  variations  of  the  angle  of  the  stone ;  and  sometimes  a 
plane  will  be  observed  which  emits  crimson  light  at  one  end, 
changing  to  orange,  yellow,  green,  etc.  until  the  other  end  of  the 
plane  shines  with  a  blue  light,  the  whole  forming  a  wonderfully 
beautiful  solar  spectrum  in  miniature.  I  need  scarcely  say  that 
the  colors  are  not  due  to  the  presence  of  any  pigment  but  are 
interference  colors  caused  by  minute  strisB  or  fissures  lying  in 
different  planes. " 

1  WOOD:  Physical  Optics,  161  (1911). 

2  Phil.  Mag.,  (5)  26,  256  (1888). 

3  Proc.  Roy.  Soc.,  17,  448  (1869). 


CHAPTER  XIII 
THICKNESS  OF  SURFACE  FILMS 

The  preceding  chapters  have  brought  out  the  importance  of 
films  in  colloid  chemistry  and  it  will  perhaps  be  worth  while  to 
get  some  definite  idea  of  the  thickness  of  the  films1  with  which  we 
have  been  dealing.  Oberbeck2  made  experiments  to  determine 
what  thickness  of  metal  deposited  electrolytically  on  platinum 
gave  the  same  electromotive  force  as  the  massive  metal.  He 
found  values  of  2-3/Aju  for  zinc,  1-2/i/j  for  cadmium,  and  1/^u  for 
copper.  Since  the  metal  was  probably  porous  and  was  not  burn- 
ished, these  figures  are  too  high  rather  than  too  low.  It  is  interest- 
ing to  note  that  the  lowest  value  is  obtained  with  copper,  the 
metal  which  gives  the  smallest  crystals  of  the  three.  Pring3 
determined  the  minimum  thickness  of  metal  which  would  give 
the  maximum  over-voltage.  His  values  vary  enormously,  being 
40MM  for  platinum,  5.5/z  for  zinc,  34/-1/*  for  copper,  0.16  mm  for 
nickel,  and  6.6  mm  for  lead.  Vincent4  determined  the  resistance 
of  thin  layers  of  silver  precipitated  chemically  on  glass  and  found 
that  the  specific  conductance  decreased  rapidly  after  the  film 
became  less  than  5(W  in  thickness.  These  figures  are  not  very 
significant  because  Grimm5  has  shown  that  the  resistance  de- 
creases with  time  and  also  with  pressure.  The  metal  firts  pre- 
cipitated is  colloidal  silver  to  some  extent  and  this  naturally 
throws  the  results  out. 

Patterson6  did  similar  experiments  with  platinum  films  pro- 
duced in  a  vacuum  by  cathodic  disintegration  and  obtained  con- 
stant values  for  a  thickness  of  7/i/i.  These  values  could  probably 
have  been  decreased  considerably  by  burnishing  the  platinum. 

1  FRBUNDLICH:  Kapillarchemie,  265  (1909). 

2  Wied.  Ann.,  31,  337  (1887). 

3  Zeit.  Elektrochemie,  19,  255  (1913). 

4  Ann.  Chim.  Phys.  (7)  19,  421  (1900). 

5  Drude's  Ann.,  6,  448  (1901). 

6  Phil.  Mag.  (6)  4,  652  (1902). 

314 


THICKNESS  OF  SURFACE  FILMS  315 

Wiener1  prepared  thin  silver  mirrors  and  determined  the  change  in 
phase  when  light  was  reflected  from  the  two  surfaces.  The  change 
of  phase  was  constant  after  a  thickness  of  12ju^  was  reached,  so 
this  is  the  point  at  which  a  silver  film  behaves  like  massive  metal 
under  these  conditions  of  test.  The  smallest  thickness  of  metal 
which  could  be  detected  optically  by  this  method  was  0.2^. 
Konigsberger  and  Bender2  found  values  of  about  5(W  for  gold 
and  platinum.  The  least  thickness  of  lead  peroxide  which  can 
be  detected  on  platinum  by  optical  methods  is  0.84ju/u. 

Bredig  and  Weinmayr3  found  that  a  film  of  mercury  thinner 
than  0.3-1.5/A/i  would  not  cause  the  catalytic  decomposition  of 
hydrogen  peroxide.  Devaux4  determined  the  smallest  thickness 
of  albumin  film  which  could  be  considered  as  solid  and  obtained 
values  between  0.4  and  8/i/i.  Metcalf5  found  about  Sjuju  as  the 
corresponding  value  for  peptone  films.  Unfortunately  he  gives 
no  data  as  to  the  maximum  thickness  of  peptone  films  which  can 
be  obtained  with  a  peptone  solution  and  yet  this  is  rather  an 
important  matter.  If  the  film  goes  on  growing  in  thickness  until 
practically  all  the  peptone  is  in  the  surface,  the  liquid- vapor  inter- 
face cannot  be  important  except  perhaps  just  at  the  start.  If 
the  film  stops  growing  in  thickness  while  there  is  still  peptone 
dispersed  through  the  solution,  what  is  the  deciding  factor? 
Shorter6  has  made  some  experiments  on  the  thickness  of  saponin 
films  but  he  does  not  give  any  actual  thicknesses,  merely  saying 
that  they  are  10-15  molecules  thick. 

Nagel7  shows  that  solid  surface  films  are  formed  on  solutions 
of  NiClj,  Ni(NO3)2,  CoCli,  CoSO4,  FeCl3,  FeSO4,  K4Fe(CN)6, 
MnSO4,  MnCl2,  Pb(NO3)2,  AgNO3,  and  KMn04  when  the  water 
contains  traces  of  ammonium  carbonate,  as  is  usually  the  case. 
The  thinnest  film  which  he  recognized  with  a  cobalt  salt  had  a 
thickness  of  about  12^.  In  many  cases  the  film  was  55-270/iju 
in  thickness.  With  colloidal  silver  a  film  at  least  300/i/z  in  thickness 
was  formed.  When  metals  are  polished,  the  surface  becomes 

1  Wied.  Ann.,  31,  629  (1887). 

2  Drude's  Ann.,  26,  763  (1908). 

3  Boltzmann  Festschrift,  839  U904). 

4  Jour.  Phys.  (4)  3,  450  (1904). 

6  Zeit.  phys.  Chem.,  62,  1  (1905). 

6  Phil.  Mag.  (6),  17,  560  (1909). 

7  Drude's  Ann.,  29,  1029  (1909). 


316  APPLIED  COLLOID  CHEMISTRY 

more  finely  crystalline  and  thus  constitutes  a  film  which  differs 
from  the  mass  of  the  metal.1  In  the  more  easily  flowed  metals 
the  surface  films  formed  by  polishing  them  may  vary  from  1-5/x  in 
thickness,  while  in  the  less  easily  flowed  metals  they  may  be 
0.5/*  or  less.  At  surfaces  which  have  moved  over  each  other  very 
slightly,  films  of  only  a  comparatively  few  molecules  in  thickness 
might  be  formed. 

Bodenstein  and  Fink2  estimated  the  thickness  of  the  adsorbed 
film  of  sulphur  trioxide  on  platinum  at  248°  as  1.2/x/x  on  the  ex- 
plicit assumption  that  the  density  of  the  film  was  2.  Gurwitsch3 
considers  that  the  thickness  of  the  adsorbed  liquid  on  silver  foil 
is  35,  33  and  28/i/^  for  carbon  tetrachloride,  carbon  bisulphide  and 
acetone  respectively. 

Ihmori4  determined  the  amount  of  moisture  on  different  sur- 
faces gravimetrically  and  estimated  the  thickness  of  the  water 
film  to  be  3-KV/A  on  metals  and  22juju  on  quartz.  Langmuir5 
questions  the  accuracy  of  these  results.  Ihmori  found  no  measur- 
able adsorption  with  polished  or  ignited  platinum.  With  the 
other  metals  the  amount  of  apparently  adsorbed  moisture  be- 
came quite  appreciable  if  the  metal  were  allowed  to  stand  in  the 
air  for  a  few  weeks  and  Ihmori  states  expressly  that  the  small 
amount  of  water  adsorbed  by  the  polished  metals  is  perhaps  due 
to  the  presence  of  a  thin  film  of  oxide.  If  the  film  of  oxide  is 
porous,  as  it  probably  would  be,  its  surface  is  vastly  different 
from  that  postulated  by  Ihmori  in  his  calculations.  Langmuir 
points  out  that  with  glass  there  is  evidence  that  water  may  pene- 
trate quite  a  way.  If  a  40  watt  incandescent  lamp  bulb,  which 
has  been  exhausted  and  dried  at  room  temperature,  is  heated  to 
500°  for  several  hours,  it  loses  enough  gas  to  form  a  layer  of 
water  55  molecules  deep  (0.00225cc/cm2)  of  carbon  dioxide  4.8 
molecules  deep  (0.00015  cc/cm2) ;  and  of  nitrogen  0.9  molecules 
deep  (0.000015cc/cm2)  all  over  the  surface.6  Since  there  can- 
not possibly  have  been  this  amount  of  gas  on  the  surface  of  the 

1  BEILBY:  Phil.  Mag.  (6)  8,  258  (1904). 

2  Zeit.  phys.  Chem.,  60,  1  (1907). 

a  Jour.  Russ.  Phys.  Chem.  Soc.,  47,  805  (1915). 

4  Wied.  Ann.,  31,  1006  (1887). 

5  Jour.  Am.  Chem.  Soc.,  38,  2284  (1916). 

6  LANGMUIR:  Trans.  Am.  Inst.  Electr.  Eng.,  32,  1921  (1913). 


THICKNESS  OF  SURFACE  FILMS  317 

bulb,  most  of  it  must  have  come  from  the  mass  of  the  glass. 
Katz1  found  that,  with  70  percent  humidity,  powdered  quartz 
takes  up  1.3  X  10~3  mg  of  water  and  anorthite  6.2  X  10~3  mg  per 
square  centimeter.  This  corresponds  to  layers  of  water  43  and 
205  molecules  deep  respectively.  Langmuir  believes  that  there 
either  was  penetration  of  water  into  the  solids  or  that  Katz  was 
measuring  moisture  condensed  as  liquid  in  the  capillary  spaces 
between  the  fine  grains.  Sherwood2  finds  that  glass  which  has 
been  annealed  above  200°  adsorbs  gases  to  give  a  layer  about 
one  molecule  thick. 

From  their  experiments  on  the  heats  of  adsorption  of  vapors 
by  charcoal,  Lamb  and  Coolidge3  conclude  that  the  adsorbed 
liquids  are  present  in  films  many  molecules  deep.  With  carbon 
bisulphide  the  thickness  of  the  layer  was  forty  molecules,  as- 
suming 0.1  HP  as  the  thickness  of  a  molecular  layer.  In  the  ex- 
periments with  ethyl  alcohol,  the  same  calculation  gives  a  layer 
about  one  molecule  thick. 

Pettijohn4  found  about  128/zAi  for  the  maximum  thickness 
of  a  water  film  on  some  glass  pearls  made  in  Germany.  With 
another  lot  of  glass  of  different  origin  the  corresponding  value  was 
about  68^At.  With  river  sand  the  estimated  thickness  of  the 
film  varied  from  285/x/i  with  10-mesh  sand  to  114^/x  with 
60-mesh  sand.  Experiments  with  nitrobenzene,  aniline,  dime- 
thylaniline,  phenyl  iodide,  toluene,  turpentine,  and  pyridine 
indicated  that  the  thickess  of  the  film  is  independent  of  the 
liquid  used,  a  result  which  can  hardly  be  strictly  true. 

Parks5  finds  that  when  glass  wool  at  12°  has  a  surface  film  of 
water  134/i^c  in  thickness,  further  addition  of  water  causes  no 
evolution  of  heat.  From  data  by  Barus6  on  the  formation  of  fog 
he  calculates  that,  under  the  conditions  of  the  experiments,  the 
thickness  of  the  water  film  is  0.1-0.8ju  when  the  nucleus  has  a 
diameter  of  2.6ju  and  is  0.05-0.5/u  when  the  nucleus  has  a  diame- 
ter of  3.6/,i.  According  to  J.  J.  Thomson7  the  mean  radius  of 

1  Proc.  Amsterdam  Acad.,  16,  445  (1912). 

2  Jour.  Am.  Chem.  Soc.,  40,  1645;  Phys.  Rev.  (2)  12,  448  (1918). 

3  Jour.  Am.  Chem.  Soc.  42,  1168  (1920). 

4  Ibid.,  41,  477  (1919). 

*  Phil.  Mag.  (6)  6,  517  (1903). 

6  Ibid.  (6)  4,  24,  262  (1902). 

7  The  Discharge  of  Electricity  through  Gases  (1900). 


318  APPLIED  COLLOID  CHEMISTRY 

drops  formed  on  negatively  electrified  oxygen  is  about  0.8/*  and 
on  positively  electrified  oxygen  is  about  0.7/*. 

Sohncke1  believed  that  films  of  oil  on  water  were  instable  when 
less  than  lOO/^  in  thickness.  This  does  not  mean  that  thinner 
films  cannot  be  obtained;  but  merely  that  this  thickness  is 
necessary  to  give  a  stable  film  having  the  properties  of  oil  in 
mass.  Fischer2  worked  with  films  of  oil  on  mercury  less  than 
5/iyu  in  thickness;  but  these  do  break  up  into  drops  in  time.  Lord 
Rayleigh3  found  that  a  layer  of  oil  1/i/x  in  thickness  causes  a 
perceptible  change  in  the  surface  tension  of  water  and  that  a 
layer  of  1.6/j/A  in  thickness  contaminates  the  water  sufficiently 
to  affect  the  motion  of  a  piece  of  camphor  over  it.  Even  with  a 
layer  of  castor  oil  8/x/x  in  thickness,  the  surface  tension  was  still 
dropping.  Langmuir4  considers  that  when  a  drop  of  oil  is 
allowed  to  spread  over  a  large  surface  of  water,  it  will  do  so  until 
the  layer  is  one  molecule  thick.  This  is  very  possibly  true;  but 
Langmuir  gives  himself  a  certain  amount  of  leeway  because  he 
admits  the  possibility  of  the  molecules  of  oil  being  packed  on  the 
surface  in  different  ways.  He  is  not  interested  at  all  in  the 
maximum  thickness  of  the  film. 

Hatschek5  estimates  the  thickness  of  the  adsorbed  water  film 
on  particles  of  colloidal  sulphur  at  0.87/x/i.  In  the  case  of  cata- 
phoresis,  Cotton  and  Mouton6  believe  that  the  effect  of  the  walls 
is  manifest  to  a  distance  of  25juM- 

The  black  film  in  a  soap-bubble  is  apparently  about  17ju/i  in 
thickness7  when  there  is  no  sodium  nitrate  in  solution  and  about 
12/xju  in  thickness8  when  there  is.  If  this  difference  is  real  it  is 
because  the  film  is  less  hydrous  in  presence  of  sodium  nitrate. 
At  higher  temperatures  the  thickness9  may  drop  to  SMA*.  I  have 
not  been  able  to  find  any  statement  as  to  the  maximum  thickness 
of  a  soap-bubble  film;  but  it  is  certainly  not  less  than  1.4ju. 

1  Wied.  Ann.,  40,  345  (1890). 

2  Ibid.,  68,  414  (1899). 

3  Phil.  Mag.  (5)  48,  331  (1899). 

4  Jour.  Am.  Chem.  Soc.,  39,  1848  (1917). 
*  Zeit.  Kolloidchemie,  11,  280  (1912). 
6NoBDENSON:  Kolloidchemische  Beihefte,  7,  108  (1915). 

7  DRUDE:  Wied.  Ann.,  43,  158  (1881). 

8  REINOLD  and  RUCKER:  Phil.  Trans.  174,  645  (1883). 

9  JOHONNOT:  Phil.  Mag.  (5)  47,  501  (1899);  (6)  11,  746  (1906). 


THICKNESS  OF  SURFACE  FILMS  319 

When  studying  reaction  velocity  in  heterogeneous  systems,  it 
has  been  found  desirable  in  many  cases  to  assume  the  existence 
of  a  film  of  saturated  solution  in  contact  with  the  solid,  the  rate 
of  reaction  depending  on  the  rate  of  diffusion.  The  thickness  of 
this  film  can  be  varied  a  great  deal  by  varying  the  rate  of  stirring. 
Under  the  conditions  in  which  Brunner1  was  working  the  thick- 
ness of  these  films  was  20-50ju. 

From  this  outline,  it  is  evident  that  the  thinnest  films  which  can 
be  detected  at  present  are  about  0.2-1. Ojuju  in  thickness,  which 
corresponds  fairly  well  to  a  layer  one  molecule  thick,  although 
there  is  no  apparent  reason  why  one  should  not  detect  a  film 
before  it  covers  the  surface  completely  and  it  is  probable  that  one 
does  in  a  good  many  cases.  The  thickness  at  which  a  film  has  the 
properties  of  matter  in  mass  may  vary  from  1-3/i/*  in  the  case  of 
metals,  up  to  much  higher  values  in  the  case  of  oils.  The  maxi- 
mum thickness  to  which  films  can  grow  and  still  be  classed  as 
films  has  not  been  determined  in  any  case,  except  perhaps  for  the 
metals;  but  it  may  very  likely  run  up  to  100/*  in  special  cases. 
There  are  certain  matters  in  regard  to  the  regions  of  discontinu- 
ity in  soap-bubble  films  which  call  for  a  good  deal  more  study. 
Nobody  seems  to  have  considered  in  any  suitable  way  the  ques- 
tion of  the  varying  hydrolysis  with  varying  thickness  of  the  film. 

Perrin2  gives  the  following  estimated  values  in  /*ju  for  the  diame- 
ters of  some  molecules:  helium,  0.17;  argon,  0.27;  mercury, 
0.28;  hydrogen,  0.20;  oxygen,  0.26;  nitrogen,  0.27;  chlorine,  0.4; 
ether  0.6. 

1  Zeit.  phys.  Chem.,  47,  56  (1904). 

2  Kolloidchemische  Beihefte,  1,  265  U910). 


AUTHOR  INDEX 


Abegg,  177 

Acheson,  173 

Adams,  70,  155 

Adkins,  46 

Aganin,  134 

Aitken,  81,  202,  276,  290,  297,  298 

Alexander,  151,  231,  297 

Alleman,  13 

Allen,  E.  T.,  152 

Allen,  H.  S.,  23,  24 

Amann,  176 

Amberger,  173,  275 

Andersen,  205 

Anderson,  249,  250,  274 

Antony,  235 

von  Antropoff,  44 

Appleyard,  105,  106 

Archibald,  23 

Arisz,  76,  194,  195,  251,  252 

Arndt,  13 

Arnold,  243 

Askenasy,  77 

Audouin,  285 

Ayres,  186 

Bachmann,  240,  241,  245,  249,  256, 

264,  274 

Bacon,  A.  A.,  147 
Bacon,  N.  T.,  13 
Bahntje,  210 
Baker,  C.  J.,  12 
Baker,  W.,  205 
Baly,  43 
Barker,  130 
Bartell,  109,  110 
Barus,  88,  167,  279,  283,  317 
Basseches,  234 
Bayer,  163 
Bayley,  92 
Bayliss,  122,  187 
Beadle,  28 
21 


Beard,  310 

Bechhold,    184,  186,  227,  234,  240, 

254,  258,  259,  271,  275 
Beck,  297 
Becker,  F.,  301 
Becker,  G.  F.,  305 
Bedford,  126 
Beijerinck,  233 
Beilby,  83,  136,  204,  316 
Bellacci,  126 
Bellamy,  306 
van  Bemmelen,   34,   103,    106,   246, 

274 

Bender,  315 

Bennett,  C.  W.,  79,  122,  149 
Bennett,  H.  S.,  210 
Bennewitz,  23 
Bentley,  W.  A.,  284 
Bentley,  W.  B.,  169,  245 
Bergter,  16 
Berkeley,  177 
fieri,  48,  51 
Berliner,  28,  47,  50 
von  Berneck,  123 
Berry,  307 
Berthelot,  74 
Berzelius,  234,  301,  308 
von  Bezold,  196 
Bieber,  277,  278,  279 
Bigelow,  67,  109 
Billitzer,  174,  206,  209,  228 
Biltz,    104,  106,  108,  187,  226,  233, 

235,  261 

Bingham,  155,  156,  157,  190,  192 
Bishop,  283 
Bleininger,  210 
Blucher,  105,  122,  246 
Blum,  169 
Blumtritt,  11 
Bodaszewski,  142 
Bodenstein,  48,  316 
321 


322 


AUTHOR  INDEX 


Bodlander,  48 

Bogojawlenski,  122 

Bondy,  188 

Bone,  46,  62,  63,  301 

Bonsdorff,  246 

Bose,  35 

Bottger,  90,  176 

Bowman,  164 

Boyle,  16,  29 

Boys,  66 

Bradford,  259 

Bragg,  37 

Brand,  68 

Bredig,  44,  123,  125,  175,   179,   180, 

186,  205,  303,  315 
Breuer,  149 
Brewer,  137 

Briggs,  L.  J.,  19,  25,  38,  97 
Briggs,  T.  R.,  79,  87,  207,  208,  210, 

224,  261,  263,  264,  265,  266 
Brillouin,  139 
de  Broglie,  144 
Brooks,  293 
Brown,  F.  E.,  67,  131 
Brown,  R.,  137 
Browne,  34 
Brunner,  319 

de  Bruyn,  Lobry,  168,  183,  186 
Buchbock,  43 
Buchner,  76 
Budgett,  75,  82 
Bunce,  244 
Bunsen,  18,  48 
Burnham,  79,  122 
Burton,  140,141,  142,  182,  190,  212, 

223,  288 
Butschli,  274 
Buxton,  227 
Byk,  165 

Cabot,  21 
van  Calcar,  186 
Callow,  272 
Calvert,  11,  39     . 
Cameron,  105,  121 
Campbell,  A.  A.,  70 
Campbell,  F.  H.,  65 


Cartledge,  251 

Cattermole,  272 

Chamberlain,  201 

Chamot,  167 

Chance,  53,  54 

Chandler,  293 

Chaney,  8,  34,  301 

Chappuis,  26,  34 

Chatard,  177 

Chattaway,  302 

Chaudesaigues,  142 

Christiansen,  197 

Church,  170 

Chwolson,  70,  77,  187 

Clark,  67 

Claude,  14 

Claus,  54 

Clermont,  151 

Clowes,  164,  225,  267 

Coehn,  207 

Coggesha  '   21 

Cohen,  288 

Cohnstaedt,  19 

Collier,  281 

Conant,  274 

Constant,  301 

Coolidge,  27,  317 

Cooper,  255 

Corelli,  126 

Cotton,  318 

Cottrell,  286,  291,  293,  302 

des  Coudres,  83 

Coulier,  276,  283 

Courtepe"e,  179 

Coward,  301 

Craig,  11 

Crookes,  18,  19,  313 

Crum,  245 

de  la  Cruz,  Garcia,  300 

Culverwell,  142 

Cunningham,  136 

Cushman,  21,  160 

Dalladay,  150 
van  Dam,  233 
Damoiseau,  39 
Davies,  245 


AUTHOR  INDEX 


323 


Davis,  N.  B.,  38,  160 

Davis,  O.  C.  M.,  34,  104,  112,  113 

Davison,  90,  119 

Day,  152 

Deacon,  48,  53 

Deane,  193 

Debus,  42 

Defoe.  99 

Delachanal,  235 

Delafontaine,  244 

De  la  Rue,  79 

Denham,  124,  175 

Desch,  79,  179 

Devaux,  315 

Dewar,  3,  7,  9,  10,  11,  13,  14,  15,  20, 

24,  25,  26,  29,  302 
Diamond,  205 
Dietl,  100 
Ditte,  152 

Doelter,  166,  177,  223,  311 
Donau,  174 

Donnan,  130,161,  278,  279 
Donnini,  177 
Doyle,  166,  173 
Dreaper,  164 
Drucker,  19,  176 
Drude,  318 
Dudley,  114 
Duff,  154 
Duin,  217 

Dumanski,  186,  253,  258 
Dutailly,  306 

Ebell,  137 

Edgar,  35,  36 

Edison,  302 

Edson,  78 

Ehrenberg,  22,  71,  72,  80,  137,  188, 

222,  288 

Ehrenhaft,  143,  144 
Einstein,     138,    140,    141,   142,   144, 

190,  191 
Elissafoff,  209 
Ellis,  C.,  60,  126 
Ellis,  R.,  261 
Elmore,  89,  271 
Emden,  148 


Emley,  156,  157 
Endell,  153,  309 
Engelder,  41,  48 
Engels,  170 
Engler,  20,  21,  277 
Erdmann,  126 
Espil,  126 
Estrup,  115 
Euler,  187 
Evans,  107 
Exner,  139,  141 

Fahrion,  263 

Fall,  154,  164,  243 

Fanto,  268 

Faraday,  47,  203,  208 

Farmer,  223 

Farnau,  105,  122,  246 

Favre,  25 

Felgate,  125 

Fenaroli,  308 

Ferguson,  135 

Findlay,  269 

Finch,  244 

Fink,  48,  79,  80,  316 

Firth,  35,  36 

Fischer,  F.,  180,  278 

Fischer,  H.  W.,  222,  244 

Fischer,  K.  T.,  318 

Fischer,  M.  H.,  253,  254 

Fischer,  W.,  169 

Flade,  240 

Fleischmann,  186 

Foote,  78 

Fortuyn,  121 

Foster,  310 

Foulk,  165 

Franchet,  307,  309,  310 

Frankland,  280,  281 

Franklin,  186 

Free,  162,  193 

Freudenberger,  186 

Freundlich,  16,  20,  100,  107,  108, 
110,  111,  112,  114,  125,  129, 
132,  135,  162,  167,  195,  208, 
210,  216,  217,  226,  227,  269, 
280,  314 


324 


AUTHOR  INDEX 


Friedel,  151 
Friedemann,  227,  228 
Friedrich,  263 
Friedenthal,  186 
Frischauer,  223 

Gadow,  199 

Galecki,  176 

Galeotti,  167,  176 

Galpin,  166 

Garbowski,  205 

Gatin-Gruzewska,  187 

Gaudechon,  27 

Geddes,  29,  32,  34,  36 

Gerke,  144,  291 

Gengou,  172 

von  Georgievics,  100,  104,  106 

Gerum,  20,  35 

Ghosh,  23 

Giampalmo,  167 

Gibbs,  130,  131,  135,  189 

Gilbert,  87,  107,  166 

Giolitti,  186,  237 

Girardi,  274 

Gladstone,  187,  270 

Glaser,  126 

Goodchild,  198,  310 

Gordon,  92 

Gossage,  53 

Goiiy,  137,  138 

Graham,  1,  73,  113,  162,  168,  169, 

177,  187,  228,  274 
Grimaux,  168,  169,  172,  244 
Grimm,  314 
Groschuff,  238 
Grout,  160 
Grunwald,  309 
Guignet,  169 
Gurwitsch,  34,  123,  316 
Gutbier,  166,  173,  205 
Guthrie,  69 
Guttmann,  294 
Gyrlai,  245 

Haber,  23,  37,  55,  56,  162,  174,  180, 

181,  223,  278,  309 
Haecker,  202,  304 


Hallwachs,  23 

Halske,  153 

Hamlin,  312 

Hannay,  304 

Hanriot,  153 

Hantzsch,  169,  171 

Harbeck,  47 

Hardy,    82,     162,     164,     212,     223, 

241 

Hargreaves,  52 
Harkins,  67,  127,  131 
Harned,  48 
Harrison,  191 
Hart,  244 
Hartl,  271 

Hartley,  E.  G.  J.,  177 
Hartley,  W.  N.,  288,  290 
Hartwagner,  176 
Hatschek,   90,   131,   134,   136,    150, 

159,  190,  191,  192,  259,  318 
Haussmann,   172 
Hautefeuille,  148 
Havard,  153 

von  Helmholtz,  H.,  207,  208 
von  Helmholtz,  R.,  276,  277 
Hempel,  4,  7,  16,  40 
de  Hcmptinne,  47 
Henderson,  43,  44 
Henry,  46,  48 
Herman,  147 
Herty,  271 
Herz,  169 
Hilgard,  100 
Himmelbaur,  174 
Hitchcock,  170 
Hober,  162,  223 
van't  Hoff,  140 
Hofmann,  A.  W.,  61,  113 
Hofmann,  F.  B.,  68,  72,  83,  84,  85, 

86 

Hofmann,  K.  A.,  61 
Hofmeister,  256 
Hohenegger,  20 
Hoitsema,  36 
Holmes,    154,    164,    165,    243,    250, 

259 
Holt,  35,  36 


AUTHOR  INDEX 


325 


Homfray,  29,  34 
Hooker,  254 
Hopkins,  164,  199 
Horner,  311 
Horsin-Deon,  244 
Hostetter,  154,  J64 
Houlehan,  34 
Hulett,  13,  74,  148 
Humphreys,  285 
Hunt,  294 
Hunter,  F.  W.,  67 
Hunter,  J.,  3,  6,  7,  16 
Hurter,  44,  53,  299 
Hutchings,  302 

Iddings,  310 
Ihmori,  18,  316 
Ikeda,  123 
Iljin,  139 
Ingraham,  169 
Ipatieff,  41,  48 
Ives,  21,  64 

Jablczynski,  125 

Jacquemin,  121 

Jamin,  77 

Jannetz,  151 

Jellinek,  56 

Jobling,  52,  53,  54,  "56,  57,  58,  60, 

62,  64,  126 
Johnson,  20,  41 
Johnston,  70,  155,  164 
Johonnot,  318 
Jones,  Chapman,  205 
Jones,  M.,  148 
Jones,  W.  J.,  136 
von  Jonstorff,  148 
Jordis,  167 
Jorissen,  223 
Jowitschitsch,  178 
Juday,  66,  282 
Jullion,  49 
Just,  278 

Kaempfer,  114,  125 
Kahlbaum,  154 
Kahlenberg,  109 


Kaiser,  66 

Karl,  126 

Karrer,  21,  64 

Kastle,  49,  124 

Katz,  256,  317 

Kaufmann,  250 

Keane,  205 

Kellner,  94 

Kelvin,  146 

Kingsbury,  21,  64 

Kirchman,  69,  150 

Klein,  169 

Klemp,  245 

Knietsch,  22,  49,  51 

Knoblauch,  294 

Kogonossow,  183 

Kohlrausch,  208 

Kohlschutter,  152,  302,  303 

Kolb,  Ellsworth,  137 

Kolb,  Emery,  137 

Koller,  73,  77,  151 

Konigsberger,  315 

Koppen,  48 

Kratz,  177 

Kressman,  77 

Krohnke,  261 

Krukenberg,  199 

Kriiss,  235 

Krutwig,  52 

Kruyt,  217 

Kugelmann,  57 

Kundt,  18 

Kuriloff,  244 

Kurzmann,  225 

Kiister,  104,  106 

Kutscheroff,  180 

Lachaud,  170 
Lachs,  103,  106,  114,  115 
Lagergren,  34,  108 
Lake,  73,  111,  113,  115,  118 
Lamb,  A.  B.,  8,  27,  34,  317 
Lamb,  H.,  208 
Landis,    8 
Lang,  270 

Langmuir,   37,   38,   39,   40,   41,   47, 
316,  317,  318 


326 


AUTHOR  INDEX 


Laplace,  134,  146 

Larguier  des  Bancels,  167 

Larsen,  194 

Lassar-Cohn,  122 

Laufer,  154 

Laurie,  310 

Lauth,  306 

lea,  98,  173,  203,  205 

Lear,  70 

LeBlanc,  61,  184 

LeChatelier,  23,  236,  237 

Lecoq,  174 

Lefebure,  20 

Lefort,  170,  172 

Lehmann,  142 

Leighton,  18,  77,  92,  93,  94 

Leith,  305 

Lenard,  23,  134,  135,  284 

Lenhardt,  285 

Leonhardt,  216 

von  Lerch,  127 

Levites,  167 

Lewis,  D.  S.,  10 

Lewis,  W.  C.  McC.,  41 

Liesegang,  258,  259 

Linder,  168,  189,  206,  221 

Liversidge,  174 

Ljubavin,  222 

Lloyd,  92,  97 

Lockemann,  91 

Lodge,  291 

Loeb,  J.  232,  253,  254,  255,  256,  267 

Loeb,  L.  B.,  28 

Loevenhart,  49,  124,  244 

Loew,  169 

Long,  180 

Lorenz,  275 

Lottermoser,  106,  167,  168,  176,  187, 

222 

Love,  301 
Lowenstein,  250 
Lucas,  152 
Lucchesi,  235 
Lucke,  62,  63 
Luckiesh,  200,  297 
Lueger,  158 
Lumiere,  228 


Lunge,  47 

Luppo-Cramer,  170,  205,  302 

Luzi,  301 

McBain,  29,  34,  100,  210,  225 

McConnel,  281,  28£ 

McDonald,  150 

Mclntosh,  78 

McKay,  194 

McPherson,  260 

Maglia,  7,  29,  34 

Magnusson,  19 

Mahin,  169 

Mahla,  49 

Mailhe,  58 

Malcolm,  196 

Malfitano,  184 

Mannich,  56 

Manville,  13 

Mar,  165 

Marc,  99,  100 

Marek,  175 

Margules,  23 

Marshall,  J.,  235 

Marshall,  J.,  275 

Martin,  183 

Martine,  26 

Masius,  114 

Masson,  A.,  179 

Masson,  O.,  25,  28,  29,  34,  76 

Mathers,  73 

Mathieu,  109 

Mat  win,  17 

Maxted,  47 

Maxwell,  297 

Mecklenburg,  167,  221,  287 

Megigrecianu,  176 

Meigen,  126 

Meissner,  78 

Melloni,  179 

Melsens,  9,  26 

Mendeleeff,  158 

Mermet,  235 

Merton,  20 

Merwin,  154,  246 

Metcalf,  189,  315 

Meunier,  168 


AUTHOR  INDEX 


327 


Meyer,  A.,  19,5,  240,  241,  242,  243 

Meyer,  G.,  133 

Michael,  274 

Michaelis,  97,  103,  106,  114,  115 

Middleton,  218 

Miller,  260 

Millikan,  136,  144 

Mills,  121 

Mines,  219 

Mitscherlich,  24,  38 

Moissan,  152 

Mond,  35,  36 

Monier,  301 

Moore,  R.  L.,  164 

Moore,  W.  C.,  264 

Moore,  W.  L.,  282,  283,  284,  285, 

286,  287,  295,  298 
Morgan,  308 
Morley,  19 
Morris-Airey,  180 
Morsi,  16 
Mott,  205,  206 
Mouret,  166 
Mouton,  318 
Miilfarth,  18 
Mulhauser,  137 
Muller,  A.,  92 
Muller,  A.,  168,  169,  176 
Muller,  E.,  180,  210 
Muller,  M.,  235 
Murray,  107,  108 
Mylius,  238 

Nagel,  C.  F.,  169,  170,  233,  244 

Nagel,  G.,  315 

Nahrwold,  290 

Nakamura,  66,  282 

Namias,  229 

Nauck,  270 

Navassart,  223 

Neel,  151 

Neisser,  227,  228 

Neogi,  302 

Neumann,  111 

Newbigin,  199 

Newton,  197 

Newman,  263 


Nicholas,  250 
Nichols,  295 
Nipher,  65 
Nobili,  197 
Nordenson,  223,  318 
Northcote,  170 

Oberbeck,  314 

Oden,  173,  191,  213,  215,  237 

Ogg,  216 

Oordt,  162 

Osaka,  96,  103,  108,  112,  114 

Osborne,  J.  L.,  165 

Osborne,  T.  B.,  220 

Osmond,  281 

Osterhout,  267 

Ostwald,  W.,  57,  58,  90,  110,  147, 

259 

Ostwald,  Wa.,  261 
Ostwald,  Wo.,  1,  210,  308 
Otobe,  282 
Oudemanns,  167 
Owen,  280 
Owens,  188,  193 
Oxland,  52 

Paal,  20,  35,  126,  274 

Pappada,  215,  216,  217 

Parker,  F.,  223' 

Parker,  F.  L.,  208 

Parker,  R.  G.,  150 

Parks,  26,  78,  317 

Parsons,  58 

Partington,  148    . 

Paternd,  176 

Patrick,  40,  127 

Patten,  105 

Patterson,  314 

Paucke,  91 

Pawlow,  148 

Pease,  56 

P&abon,  301 

Pelet-Jolivet,  115,  116,  118- 

P&igot,  189 

Pelouze,  285 

Perley,  58 

Perrey,  148 


328 


AUTHOR  INDEX 


Perrin,  138,  139,  140,  144,  208,  209, 

295,  319 
Pettijohn,  317 
Pfund,  198,  132 
Phillips,  F.  C.,  302 
Phillips,  P.,  49 
Pickering,  245,  262,  264 
Picton,  168,  189,  206,  221 
Pieroni,  176,  274 
Pierson,  210 
Pingree,  224 
Pirsson,  310 
Piutti,  7,  29,  34 
Plashke,  61 
Playfair,  123 
Pluddemann,  34,  111 
Pockels,  72 
Pollock,  281 
Poppe,  160 
Porlezza,  16 
Posnjak,  246,  257,  258 
Potts,  161 
Pouillet,  26 
Powis,  261 
Prat,  153 
Pring,  314 
Pringal,  277 
Prost,  168 
Prudhomme,  170 
Przibram,  279 
Publow,  77,  220 

Quincke,  160,  207,  240 

Ramage,  288,  290 

Rammelsberg,  154 

Ramsay,  13,  23,  35,  36,  245 

Ramsden,  271 

Randau,  75 

Raoult,  154 

Raschig,  43 

Rasenfosse,  174 

Rayleigh,  65,  67,  72,  81,  82,  83,  88, 
131,  132,  134,  151,  182,  197, 
202,  287,  295,  313,  318 

Recoura,  229 

Reed,  180 


Reedy,  149,  152 

Reese,  50 

Regnault-Betton,  204 

Reid,  E.  E.,  16 

Reid,  E.  W.,  179,  187 

Reinders,  86,  87,  98,  165 

Reinold,  318 

Reinsch,  91 

Reitlinger,  23 

Reusch,  304 

Reuss,  207 

Reychler,  20,  36 

Reyerson,  293 

Reynolds,  66,  158,  159 

Rhead,  12,  37 

Rice,  75 

Richards,  28,  76 

Richardson,  C.,  123,  275 

Richardson,  L.  B.,  31,  32 

Richarz,  276,  277 

Rideal,  46,  49,  52,  54,  55,  56 

Riffard,  168,  172 

Rindfusz,  164 

Ringer,  244 

Rittenhausen,  173 

Ritter,  284 

Roberts,  280 

Robinson,  F.,  52 

Robinson,  I.,  24 

Rodewald,  257,  258 

Roessler,  175 

Rohland,  20 

Rona,  97,  114 

Rontgen,  129,  136 

Rose,  G.,  70 

Rose,  R.  P.,  169,  237,  245 

Rose,  T.  K.,  235 

Rosenhain,  303,  307 

Rosinger,  275 

Rosnyak,  125 

Rothe,  106 

Rothmund,  23,  280 

Rubenbauer,  169 

Rucker,  318 

Rudge,  294 

Ruer,  23 

Ruff,  153 


AUTHOR  INDEX 


329 


Russ,  163 
Huston,  288 
Rutherford,  15 

Sabaneeff,  187 

Sabatier,  41,  44,  57,  58,  59,  60,  126 

Sabin,  21 

Sack,  180 

Sahlblom,  92 

Saida,  126 

Sakulka,  66 

Salamon,  54 

Sansonoff,  174 

Satterly,  16 

de  Saussure,  7 

Saxton,  78 

Schade,  312 

Schaposchnikoff,  122 

Scheermesser,  20 

Scheetz,  205 

Schenck,  206,  312 

Schiaparelli,  170 

Schiff,  167 

Schlaepfer,  264 

Schloesing,  22,  223 

Schmauss,  285 

Schmidt,G.  15, 100, 102, 106, 110, 114 

Schmidt,  W.  A.,  293 

Schmidt,  Walter,  104 

Schneider,  E.  A.,  274 

Schneider,  J.,  129 

Schoep,  184 

Schonbein,  47,  123 

Schoop,  303 

Schottlander,  173 

Schraube,  13 

von  Schroeder,  75,  76,  110,  177 

Schucht,  107,  217 

Schultze,  71,  72,  80 

Schulze,  112,  209,  213,  217,  218,  223, 

228,  308 
Schuster,  22 
Schutzenberger,  301 
Schwalbe,  76 
Schwarz,  235 
Schwerin,  210 
Seager,  63 


Seddig,  142 

Seeliger,  285 

Seger,  306 

Selmi,  1 

Senderens,  41,  57,  58 

Senter,  123,  124 

Sestini,  90 

Seyewetz,  228 

Shaw,  60 

Shenstone,  40 

Sheppard,  252,  264 

Sherrick,  94,  99,  218 

Sherwood,  18,  317 

Shields,  35,  36 

Shorter,  189,  315 

Siedentopf,  182,  311 

Siegmund,  126 

Siemens,  153 

Simcoe,  73 

Simpson,  285,  286 

Sisley,  105 

Skey,  10,  91,  114,  177,  223 

Slade,  169 

Smith,  Angus,  39 

Smith,  A.  F.,  73 

Smith,  C.  R.,  195,  224,  252 

Smith,  J.  Cruickshank,  72,  88 

Smith,  J.  Lawrence,  147 

Smoluchowski,  140,  142 

Smyth,  H.  D.,  293 

Smyth,  Piazzi,  283 

Snowdon,  149 

Sohncke,  318 

Sorensen,  255 

Sosman,  154 

Soxhlet,  170 

Spencer,  23 

Spengel,  205 

Spezia,  154 

Spring,  77,  151,  154,  168,  170,  178, 

202,  300 
Squibb,  44 
Stark,  269 
Stearn,  253 
Stefan,  74 
Steiner,  108 
Steinmetz,  64,  69 


330 


AUTHOR  INDEX 


Stepanoff,  23 

Stevenson,  249 

Stewart,  169 

Stockle,  133,  134 

Stokes,  136,  142,  211,  306,  313 

Stoney,  21 

Storey,  10 

Stritar,  268 

Strong,  175 

Strutt,  42 

Svedberg,  138,  141,  144,   169,    172, 

173,    174,   175,   176,    179,    180, 

223,  274 
Swan,  127 
Swinton,  18 
Szilard,  168 

Tait,  9 

Tammann,  81,  151,  249 
Tangl,  136 
Taylor,  F.  W.,  157 
Taylor,  H.  S.,  47,  48,  49, 52,  54,  55,  56 
Taylor,  W.  E.,  163 
Teague,  227 
Thibault,  170,  172 
Thiel,  56 
Thompson,  157 
Thomson,  J.  J.,  279,  280,  317 
Thomson,  W.,  25,  146 
Thorp,  68,  155 

Thorpe,  52,  54,  55,  178,  307,  309 
Thoulet,  91 
Tiebackx,  232 
Tinker,  109 
Titoff,  29,  31,  34 
Toch,  80 

Tolman,  253,  287,  293 
Tommasi,  122 
Travers,  29,  30,  34 
Traube,  219 
Trouton,  29,  30,  99 
Triimpler,  100 
Tubandt,  169 
Tumlirz,  74 
Twomey,  127 

Tyndall,  22,  179,  182,  195,  197,  200, 
202,  287;  298,  305 


Ullmann,  19 
Upton,  158 

Valentiner,  15 

Vanino,  175,  271 

Van  Name,  181 

Van  Slyke,  77,  103,  106,  220,  244 

Vater,  4,  7,  16 

von  Vegesack,  187 

Verneuil,  311 

Vignon,  76,  221 

Vincent,  314 

Vogel,  53 

Von  Engeln,  148,  281 

van  der  Waals,  134 

Wadsworth,  302 

Wagner,  106,  107 

Wald,  70 

Waldbott,  97 

Walden,  185 

Waldie,  223 

Walker,  A.  O.,  291 

Walker,  J.,  105,  106 

Warburg,  18 

Warington,  302 

von  Wartenberg,  23 

Washburn,  76 

Watson,  G.,  166 

Watson,  T.  L.,  310 

Weber,  71 

Wegelin,  178 

von  Weimarn,   163,   164,   165,   166, 

167,  176,  183,  238,  275 
Weingartner,  173 
Weinmayr,  315 
Weisberg,  168 

Weiser,  94,  99,  165,  218,  299 
Weiske,  92 
Welker,  235 
Wells,  144,  291 
Wetherill,  155 
Whatmough,  129 
Wheeler,  12,  37 
Wiedeburg,  77 
Wiedemann,  207,  208 
Wiegner,  105 


AUTHOR  INDEX 


331 


Wieland,  61 

Wiener,  137,  315 

Wiesner,  284 

Wigand, 176 

Wild,  21,  277 

Willard,  271 

Williams,  A.  E.,  306 

Williams,  A.  Mv  32 

Willows,  131,  134,  136,  150 

Wilson,  C.  T.  R.,  278,  279,  280,  285 

Wilson,  G.,  127,  128 

Wilson,  R.  E.,  8,  34 

Winkelblech,  260,  261,  271 

Winssinger,  168,  176 

Wintgen,  188 

Witzmann,  174 

Wohler,  F.,  49 

Wohler,  L.,  20,  34,  111,   153,    170, 

174,  205,  235 
Wohler,  P.,  34,  111 
Wolf,  278 
Wolff,  76 


Wood,  197,  199,  205,  282,  298,  313 

Worden,  88 

Worley,  129,  130 

Worthington,  66,  74 

Woudstra,  223 

Wright,  A.  W.,  302 

Wright,  F.  E.,  152 

Wright,  L.  T.,  175,  177 

Yorke,  92 

Young,  S.  W.,  168,  224 

Young,  T.,  283 

Zangger,  139 
Zawrieff,  180 
Zeidler,  270 
Zeisberg,  51 
Zlobicki,  189 

Zsigmondy,  139,  182,  206,  217,  221, 
230,  235,  239,  240,  249,  305,  306 
Zulkowski,  307 


SUBJECT  INDEX 


Abnormal  adsorption,  106-108 

end-points,  49 

Acid,  hydrochloric,  4,  9,  20,  26,  44, 
49,  50,  52,  53,  92-96,  107, 
116-119,121,127,129,165, 
175,  214,  216 
lysalbinic,  173 
nitric,  57,  58,  123,  129,  175 
picric,  and  silk,  105 
protalbinic,  173,  308 
sulphuric,  22,  49-51,  61,  92-96, 

115-119,  165,  180 
Action,  catalytic,  of  charcoal,  39,  40 

mutual,  of  sols,  225-235 
Addition  agents,  165,  231 

of  agglomerating  agent,  rate  of, 

220 
of  peptizing  agent,  sols  by,  177, 

178 

Adhesion  of  hot  powders,  81 
Adhesives,  74 

Adsorbed  gas,  density  of,  24 
vapor  pressures  of,  25 
Adsorbent,  poisoning  of,  47-53,  56- 

59,  123-125 
Adsorbing  power  of  fuller's  earth, 

121 

Adsorption,  abnormal,  106-108 
Adsorption  and  density,  70,  91 

and   hydrogen   ion   concentra- 
tion, 219 

and  hydrolysis,  120-122 
and  peptization,  166 
and  solvent,  112,  113 
and  surface  tension,  135,   166, 

167 

and  the  triple  point,  70 
and  valence,  112,  209,  213-219 
and  vapor  density,  19 


Adsorption  by  charcoal,  3-18,  24-40, 
90,  91,  96-98,  101-106,  108, 
110-114,  233 
by  crystals,  99 

by  filter  paper,  92,  107,  108,  178 
effect  of  pressure  on,  3,  9, 16,  20, 

29,  32,  36 
of  temperature  on,  3-5,  9,  13, 

29,  32,  113 
from  solution  by  liquid,  126-128 

by  solid,  90-126 
gravimetric    determination   of, 

93-97 

heat  of,  24^27,  78,  113,  258 
in  cathode  ray  tubes,  18,  19 
isotherm,  29-37,  100-107 

exponent  in,  33,  100,  106 
negative,  90,  108-110 

and   osmotic   pressure,    109, 

110 
of  dyes,  90,  104,  112,  115-120, 

232 

of  strong  electrolytes  by  char- 
coal, 96-98,  238 
of  essential  oils,  73,  332 
of  gases  by  solids,  3-64 
of  liquid  by  fibers,  76,  77 
by  liquid,  81-83 
by  solid,  67-78,  140 
of  mixed  gases,  10,  16 
of  rouge  by  metals,  79 
of  several  solutes,  113-120 
of  solid  by  liquid,  83-89,  260 

by  solid,  78-81 
of  vapor  by  liquid,  65-67,  134, 

135 

of  vapors  by  solids,  3-64 
positive,  90 
reversibility  of,  110,  111 


333 


334 


SUBJECT  INDEX 


Adsorption,  selective,  catalysis  and, 

41 

specificity  of,  111-113 
titration  determination  of,  93- 

97 

vacuum  by,  9 

Agar-agar,  74,  190,  233,  243,  274 
Agate,  198 

Agent,  agglomerating,  rate  of  addi- 
tion of,  220 

sols  by  removal  of,  177 
Agents,  addition,  165,  231 
emulsifying,  261,  264 
Agglomerating  agent,  172,  173,  177, 

220 

rate  of  addition  of,  220 
sols  by  removal  of,  177 
Agglomeration,  111,  153,   161,    162, 
170,    177,    192,    195,    212, 
220,  221,  240 
reversibility,  177,  221 
Air  films  and  mobility,  21 
and  voltage,  22,  23 
washing  of,  299 

Albumin,  1,  91,  173,  185,  187,  188, 
219,  221,  231,  260,  271,  315 
Alcohol,  solidified,  262 
Alumina,  1,  18,X20,  41,  44,  85,  91 
120,  123,  169,  170,  172,  206, 
226,  227,  245,  246,  312 
Aluminum,  1,  19,  164,  273 
Amalgam,  sodium,  69 
American  method  of  making  emul- 
sions, 265 
Ammonia,  4,  6,  16,  19,  26,  31,  43, 

55-57,  228,  277 
process,  55-57 
Amplitude,  138 
Analysis  of  smokes,  293 
Angle,  contact,  67 
Anions,  115,  213-220,  224 
Antagonism  of  ions,  267 
Aquadag,  156,  173 
Argon,  4,  6,  13,  14,  26 
Armor-plated  bubbles,  272,  273 
Arsenious  oxide,  50,  51,  53,  56,  91, 
175 


Arsenious  sulphide,   175,   185,   188, 

226-228,  260 
Asphalt,  275 

Baking  powders,  phosphate,  80 
Banding,  rhythmic,  259 
Barium  salts,   1,  92,   120,   163-165, 
172,  190,  209,  214-218,  220, 
239 

Bayer  process,  163 
Beaches,  wells  near,  99 
Beta  rays,  223 
Bishop's  ring,  283 
Bitumen,  123 
Blackness,  199 
Blue  eyes,  200-202 

feathers,  201,  202,  304 
powder,  79 
quartz,  310 
sky,  202,  295,  304 
sun,  283 
Body  color,  203 
Bombardments,  molecular,  138 
Borneol,  dehydrogenation  of,  60 
Bronzing  liquids,  87,  88 
Brownian  movements,  136-145,  183, 

186,  187 

and  size  of  particles,  139 
in  gases,  143,  144 
laws  of,  138-142 
Bubbles,  2,  66,  198,  199,  268-273, 

304,  318 
and  drops,  coalescence  of,  65, 

66,  286 

armor-plated,  272,  273 
Butter,  2,  241,  263,  266 

Calcium  soaps,  262,  263,  267,  268, 

274 

sulphate,  filtration  of,  159 
Callow  process,  272 
Cape  Cod  lighter,  68 
Capillarity,  25,  67,  69,  70,  146,  247 
Carbon  black,  21,  72,  157,  178,  264, 

270 
dioxide,  4-6,  11-13,  20,  24,  26, 

32,  37,  39,  133 


SUBJECT  INDEX 


335 


Carbon  monoxide,  4,  5,  11-13,  26, 

36,  37,  39,  46-48 

Caseine,  2,  77, 103, 163, 167, 169, 173, 
185,  191,  219,  220,  231-233 
Cassius,  purple  of,  234,  235 
Catalysis  and  degree  of  dispersion, 

125 

and  fields  of  force,  43 
and  selective  adsorption,  41 
and  viscosity,  43 
contact,  40-64,  122 
of  hydrogen  peroxide,  44,  123- 

125 

theories  of,  40-14 

Catalytic  action  of  charcoal,  39,  40 
Cataphoresis,  206,  268,  318 
Cathode  ray  tubes,   adsorption  in, 

18,  19 

Cations,  115,  213-220 
Cattermole  process,  272 
Cellulose,  35,  76,  161 

nitrates,  88,  167,  274 
Cement  dust,  293 

Portland,  and  gypsum,  80 
Centrifuging  of  sols,  186 
Ceria,  64,  226,  227 
Chance-Glaus  process,  53 
Charcoal,  adsorption  by,  3-18,  24- 
40,  90,  91,  96-98,  101-106, 
108,  110-114,  233 
adsorption    of    strong    elec- 
trolytes by,  96-98,  233 
catalytic  action  of,  39,  40 
cocoanut,  3,  4,  67 
from  different  woods,  6,  8 
gas-treated,  301 
oxidation  of,  10-13 
structure  of,  7 
synthetic,  5 
Charge  on  diaphragms,  207-209 

reversal  of,  219 
Chlorine  process,  Deacon,  52 
Chloropicrin,  8,  16,  48 
Christiansen  effect,  197 
Chromic  oxide,  111,  123,  168,  170, 
184,     226-230,     233,     234, 
246,  311 


Chromite  and  magnetite,  69 
Clay,  plasticity  of,  160 
Clouding  of  gels,  247-250,  256 
Coalescence  of  drops  and  bubbles, 
65,  66,  286 

of  liquids,   146,   147,   161,  261, 
265 

of  solids,  147-154,  161 
Coarse  and  fine  powders,  79 
Cocoa,  190 

Cocoanut  charcoal,  3,  4,  6,  7 
Coherer,  23 
Collodion  membranes,  184,  240 

pores  in,  185,  240 
Colloid,  definition  of,  1 

peptization  by,  167,  170 

stabilization  by,  212,  225 
Colloidal  fuel,  275 

ice,  melting  of,  78 

solutions,  true  and,  144,  183 
Colloids,  color  of,  196-206 

protecting,  170-173,  180 
Color,  body,  203 

of  coUoids,  196-206 

of  gems,  224,  311,  312 

of  metal  sols,  203-205,  308 

of  powders,  198  • 

of  rouge,  205 

of  water,  202,  203 

pigment,  196 

structural,  196 

surface,  203 
Colors,  sunset,  283 
Combustion,  surface,  62 
Comets,  297 

Composition  of  dust,  288-290 
Compounds,  indefinite,  39 

intermediate,  44 

stabilization  of,  122 
Concentration  and  surface  tension, 
130,  131 

surface,  130-132 
Condensation  methods,  172-176 
Conductance  of  emulsions,  267 

of  sols,  210 

without  solutes,  211 
Consolidation  settling,  193 


336 


SUBJECT  INDEX 


Contact  angle,  67 

catalysis,  40-64,  122 
sulphuric  acid,  49-51 
Continental  method  of  making  emul- 
sions, 265,  266 

Copper,  20,  59,  60,  79,  150,  173,  174, 
180,  272,  275,  278,  306,  312, 
314 

ferrocyanide  membranes,  110 
oxide,    52,    107,    122,    168-170, 

172,  173,  206,  246 
Coronas,  282,  283 
Cotton,  27-30,  76,  77,  92-96,  108, 

116,  118-120,  147 
Cottrell  process,  286,  291-293 
Cracking  of  emulsions,  268 
Cream,  186,  194,  268,  271 
Crystallization    of    gels,    107,    166, 

223 

Crystals,  adsorption  by,  99 
filamentous,  302 
size  of,  164 
Crystal  size  and  melting-point,  148 

and  solubility,  148 
Cubical  piling,  157,  159 
Curdy  precipitates,  162,  163,  231 

Daguerreotype,  73 
Deacon  chlorine  process,  52 
Decomposition  of  esters,  45,  46 

of  vermilion,  79 

photochemical,  sols  by,  176 
Definition  of  colloid,  1 
Dehydrogenation,  60 

of  borneol,  60,  61 
Density  and  adsorption,  70,  91 

of  adsorbed  gas,  24 

of  smokes,  300 

of  sols,  188 

vapor,  adsorption  and,  19 
Detection  of  strychnine,  113 
Determination,  gravimetric,  of  ad- 
sorption, 93-97 

of  type  of  emulsion,  263 

titration,  of  adsorption,   93-97 
Diagram  for  silica,  248 
Dialysis,  188,  243 


Diameter  of  molecules,  319 
Diaphragms,  charge  on,  207-209 

flow  through,  206-210 
Diffusion,  164,  187,  258 
Dilatancy,  158,  159 
Dineric  interface,  83,  260 
Discharge,  oscillatory,  180 
Discontinuity,  surface  tension  and, 

134 
Disintegration,  electrical,  sols  by,  179 

electrochemical,  sols  by,  180 

mechanical,  sols  by,  161,  178 
Dispersed  phase,  1,  2 
Dispersing  phase,  1,  2 
Dispersion,  degree  of,  catalysis  and, 
125 

methods,  172,  177-180 
Drops,  2,  65,  66,  71,  146,  148,  161, 
163,  242,  261,  265,  282 

and  bubbles,  coalescence  of,  65, 
66,  286 

electrification  of,  65 

optical  properties  of,  282,  283 

rain,  1,  283-285 

rain,  size  of,  284-286 

rolling,  66,  71,  282 
Dry  fog,  280,  281 
Drying,  electrical,  210 
Dust,  287-299 

cement,  293 

composition  of,  288-290 

electrification  of,  294 

from  Krakatoa,  288,  296 

meteoric,  288-290 

optical  properties  of,  295-297 
Dyes,   adsorption  of,  90,   104,   112, 

115,  120,  232 

Dynamic  and  static  surface  tensions, 
131-134 

Effect,  Christiansen,  197 

of  gravity  on  sols,  139,  186 

of  ions  on  syneresis,  244 

of  pressure  on  adsorption,  3,  9, 

16,  20,  29,  32,  36 
of  temperature  on  adsorption, 

3-5,  9,  13,  29,  32,  113 


SUBJECT  INDEX 


337 


Electrical  disintegration,  sols  by,  179 

drying,  210 

endosmose,  206-210 

properties  of  sols,  206-212 

tanning,  210 
Electrification  of  drops,  65 

of  dust,  294 

of  ice,  302 
Electrochemical  disintegration,  sols 

by,  180 

Electrolysis,  sols  by  reduction  with- 
out, 173 

Electrolytes,  adsorption  of,  by  char- 
coal, 96-98,  233 

Electrolytic  reduction,  sols  by,  174 
Electromotive  force   measurements 

with  sols,  206,  211,  212 
Electrons,  emission  of,  278 
Elmore  bulk  oil  process,  89,  271 
Elutriation,  136 
Emanation,  15,  16,  114,  311 
Emission  of  electrons,  278 
Emulsifying  agents,  261,  264 
Emulsion  jellies,  240,  264 
Emulsions,   2,    161,   240,   241,   260- 
268,  275 

conductance  of,  267 

cracking  of,  268 

intermittent  shaking  of,  265 

making,   American  method  of, 

265 

Continental  method  of,  265, 
266 

photographic,  231 

type  of,  determination  of,  263 

types  of,  261 
Enamels,  307,  309 
Endosmose,  electrical,  206-210 
End-points,  abnormal,  49 
Enzymes,  48,  49 

Essential  oils,  adsorption  of,  73,  232 
Esters,  decomposition  of,  45,  46 
Ethylene,  17,  41,  45 
Exponent   in    adsorption   isotherm, 

33,  100,  106 

External  phase,  1,  2,  166,  187,  263 
Eyes,  blue,  201,  202 

22 


Fabrics,  shower-proof,  71 

Fatigue,  photo-electric,  23 

Fats,  hydrolysis  of,  268 

Feathers,  blue,  201,  202,  304 
frost,  282 

Ferric  oxide,  51,  52,  79,  87,  91,  111, 
168,  172,  175,  185,  187- 
189,  205,  217,  218,  221, 
226-228,  232-234,  238,  245, 

270,  310 
Ferrocyanide,    copper,    membranes, 

110 
Fibers,  adsorption  of  liquid  by,  76, 

77 

Fields  of  force,  catalysis  and,  43 
Filamentous  crystals,  302 
Filaments,  2,  37,  42,  47,  240,  245, 

302 

Films,  2,  20-23,  25,  38,  65-67,  81, 
82,  89,  128,  151,  160,  189- 
191,  195,  260,  269,  271, 
314-319 

air,  and  mobility,  27 
and  voltage,  22,  23 
liquid,   67,   81,   128,    160,    191, 

269,  315-319 
solid,   89,    189,    190,    195,   260, 

271,  314-316 

thickness  of,  25,  38,  82,  160,  189, 

191,  314-319 
Filter  paper,  adsorption  by,  92,  107, 

108,  178 

Filtration  of  calcium  sulphate,  159 
Fine  and  coarse  powders,  79 
Flames,  174,  299,  300 
Flamingoes,  199 
Flocculation,  105,  106 
Flocculent  precipitates,  162,  163 
Floridin,  123 
Flotation,  2,  271-273 
Flow  through  diaphragms,  206-210 
Fluidity,  154,  210 

zero,  156,  157,  190,  192,  240 
Foam,  268-273,  304 

stabilization  of,  270,  272 
Fog,  1,  275-286,  298 

-bows,  282 


338 


SUBJECT  INDEX 


Fog,  dry,  280,  281 

precipitation  of,  285 

supercooled,  281 
Fogs,  metallic,  275 
Force,  fields  of,  and  catalysis,  43 
Formaldehyde,  61,  184,  225 
Freezing  of  sols,  78 
Fritting,  151 
Frost  feathers,  282 
Froth,  268,  269,  272 
Fuel,  colloidal,  275 
Fuller's  earth,  adsorbing  power  of, 
121 

Gamboge,  139-142 

Gas,  adsorbed,  density  of,  24 

illuminating,  purification  of,  17 

mantles,  54 

vapor  pressures  of  adsorbed,  25 
Gases,  adsorption  of,  by  solids,  3-64 

Brownian  movements  in,   143, 
144 

in  solids,  304,  305 

mixed,  adsorption  of,  10,  16 
Gas-treated  charcoal,  301 
Gelatine,   1,  73-76,   161,   165,   168, 
172,    174,    178,    184,    185, 
188,  189,  194,  195,  204,  224, 
225,  228-233,  251-260,  274 

jellies,  vapor  pressures  of,  75, 
256 

strains  with,  73 
Gelatinous    precipitates,    162,    163, 

177,  236-240 
Gels,  162 

clouding  of,  247-250,  256 

crystallization  of,  107,  166,  223 

structure  of,  236-245 

vapor  pressures  of,  236,  245-250 
Gems,  color  of,  224,  311,  312 
Glass,  ruby,  2,  305-307 

welding  of,  .149,  150 
Gold  leaf,  granulation  of,  83 

number,  230,  231 
Grains,  2,  83,  148,  161 
Granulation  of  gold  leaf,  83 
Graphite,  2,  44,  156,  190,  192,  240 


Gravimetric  determination  of  ad- 
sorption, 93-97 

Gravity,  effect  of,  on  sols,  139,  186 
welding  by,  151 

Gum  arabic,  68,  170,  172,  179,  231, 
232,  265,  266,  269 

Gypsum  and  Portland  cement,  80 

Haemase,  poisoning  of,  123,  124 

Hail,  286,  287 

Halos,  283,  298 

Harmattan,  296 

Haze,  297,  298 

Heat  of  adsorption,  24-27,  78,  113, 
258 

Helium,  4,  5,  9,  13,  14,  24 

Hematite,  205 

Hexagonal  piling,  157,  159 

Honeycomb  structure,  163,  239,  264 

Hot  powders,  adhesion  of,  81 

Hydrochloric  acid,  4,  9,  20,  26,  44, 
49,  50,  52,  53,  92-96,  107, 
116-119,121,127,129,165, 
175,  214,  216 

Hydrogels,  162 

Hydrogen,  4,  5,  9,  13,  14,  17,  23-26, 
28-31,  35,  37,  40,  41,  45-48, 
58-61,  126,  133 

Hydrogenation,  58-60,  126 
of  oils,  60,  126 

Hydrogen  ion  concentration,  adsorp- 
tion and,  219 

peroxide,  catalysis  of,  44,  123- 
125 

Hydrolysis,  adsorption  and,  120-122 
of  fats,  268 
sols  by,  175 

Hydrophane,  304 

Hydrosols,  162 

Ice,  colloidal,  melting  of,  78 

cream,  231 

electrification  of,  302 
Illuminating  gas,  purification  of,  17 
Indefinite  compounds,  39 
Index,  refractive,  188,  199 
Indigo,  21,  123,  203 


SUBJECT  INDEX 


339 


Inhomogcneity  of  sols,  182 
Ink,  1,  92,  178 
Interface,  dineric,  83,  260 


Leaf,  gold,  granulation  of,  83 
Leaking  of  tents,  71 
Liesegang's  rings,  258,  259 


Interlacing  systems,  2,  239,  242,  243,      Light,    ultra-violet,    223,    278,    280, 

311,  312 
Liquid,  adsorption  from  solution  by, 

126-128 

adsorption  of,  by  fibers,  76,  77 
by  solid,  67-78,  140 
liquid  by,  81-83 
solid  by,  83-89,  260 
vapor  by,  65-67,  134,  135 
films,  67,  81,  128,  160,  191,  269, 

315-319 

peptization  by,  167,  224,  230 
Liquids,  bronzing,  87,  88 

coalescence  of,   146,    147,    161, 

261,  265 
in  sieves,  70,  72 
tensile  strength  of,  74 


304 

Intermediate  compounds,  44 
Intermittent  shaking  of   emulsions, 

265 

Internal  phase,  1,  2,  166,  263 
Iodides  in  sea-water,  90 
Iodine,  90,  98,  104,  111,  147,  277,  278 

starch,  104,  111 
Ion,  peptization  by,  167,  168 
Ions,  antagonism  of,  267 

effect  of,  on  syneresis,  244 

stabilization  by,  212-223 
Iron,  36,  56,  72,  122,  154,  180 

parkerized,  72 

passive,  122 
Irregular  series,  227 


Isotherm,    adsorption,    29-37,    100-      Lloyd's  reagent,  97 


107 


Lycopodium  powder,  68,  80,  270 


adsorption,    exponent    in,    33,      Lysalbinic  acid,  173 
100,  106 


Itacolumite,  155 

Jasper,  310 

Jelly,  2,  163,  236,  237,  240-259,  262, 

274 

emulsion,  240,  264 
gelatine,  vapor  pressures  of,  75 

256 

stabilization  of,  244 
swelling  of,  243,  251-256 

Kenotron,  293 

Kerosene,  68,  72,  84,  85,  87,   190, 

261,  262,  264 

Krakatoa,  dust  from,  288,  296 
Krypton,  15 

Lamination,  305,  312 

Law,  Schulze's,  112,  209,  213-219 

Stokes',  136,  142,  211 
Laws  of  Brownian  movements,  138- 

142 
Leaching  of  whiskey,  114 


Magenta,  121,  203 
Magnesium  soaps,  262,  268,  274 
Magnetite  and  chromite,  69 
Making  emulsions,  American  method 

of,  265 
Continental  method  of,  265, 

266 
Mantles,  gas,  64 

Welsbach,  64 
Marksmen,  201 
Mayonnaise,  264,  266 
Mechanical  disintegration,  sols  by, 

178 

Meerschaum,  20,  305 
Melting  of  colloidal  ice,  78 
Melting-point,  crystal  size  and,  148 
Membranes,  collodion,  184,  240 

pores  in,  185,  240 
copper  f  err  ocyanide,  110 
semipermeable,  109 
Mercury,  24,  44,  68-70,  72,  73,  89, 

128,  133-135 
Metallic  fogs,  275 


340 


SUBJECT  INDEX 


Metal  sols,  color  of,  203-205,  308 
Metals,  adsorption  of  rouge  by,  79 
Metathetical  reactions,  sols  by,  175 
Meteoric  dust,  288-290 
Methane,  5,  17,  39,  45,  46 
Method,  American,  of  making  emul- 
sions, 265 

Continental,   of  making  emul- 
sions, 265,  266 
Methods,  condensation,  172-176 

dispersion,  172,  177-180 
Mica,  74,  151 
Micelles,  211 

Migration  velocities,  206,  210 
Mirrors,  302 

Mixed  gases,  adsorption  of,  10,  16 
paints,  80,  156,  262 
solvents,  peptization  by,  167 
Mobility,  air  films  and,  21 
Molecular  bombardments,  138 

weights,  141,  145,  187 
Molecules,  diameter  of,  319 
Monoxide,  carbon,   4,  5,  11-13,  26, 

36,  37,  39,  46-48 
Mordant,  90,  112,  122 
Mother-of-pearl,  198,  311 
Movements,     Brownian,     136-145, 

183,  186,  187 
and  size  of  particles,  139 
in  gases,  143,  144 
laws  of,  138-142 
Mutual  action  of  sols,  225-235 

Negative  adsorption,  90,  108-110 

and  osmotic  pressure,  109, 110 
Neon,  13,  14 
Newton's  rings,  197 
Nickel,  41,  43,  45,  58-61,  79,  125, 

126,  149 
Nitric  acid,  57,  58,  123,  129,  175 

process,  57 
Nitrogen,  4-6,  10,  11,  13-16,  24,  26, 

39,  42,  43,  55-58,  133 
Nobili's  rings,  197 
Non-aqueous  sols,  230,  274,  275 
Non-electrolyte,  peptization  by,  167, 

168 


Nuclei,  276-280 
Number,  gold,  230,  231 

Oat-meal,  25,  147 

Obsidian,  310 

Oil,  2,  48,  81,   161,   189,  242,  260, 

268,  274 

Oil-in-water,  2,  262,  264,  268 
Oils,  essential,  adsorption  of,  73,  232 
hydrogenation  of,  60,  126 
spreading  of,  82 
Opacity  of  paper,  80,  199 
Opals,  304,  313 
Optical    properties    of    drops,    282, 

283 

of  dust,  295-297 
of  sols,  182,  183,  188,  193-205 
Order  of  ultra-filtration,  185 
Oscillatory  discharge,  180 
Osmium,  56 

tetroxide,  61 

Osmotic  pressure,  negative  adsorp- 
tion and,  109,  110 
pressures,    109,    110,    141,    186, 

187,  189 

Over-voltage,  23 
Oxidation  of  charcoal,  10-13 
preferential,  46 
sols  by,  174 
Oxide,  arsenious,  50,  51,  53,  56,  91, 

175 

chromic,    111,    123,    168,     170, 
184,  226-230,  233,  234,  246, 
311 
copper,  52,   107,  122,  168-170, 

172,  173,  206,  246 
ferric,  51,  52,  79,  87,  91,  111, 
168,  172,  185,  187-189,  205, 
217,    218,     221,    226-228, 
232-234,  238,  245,  270,  310 
Oxygen,  4-6,  10-16,  23,  24,  26,  36, 
37,  39,  40,  42,  44,   47-54, 
57,  58,  62-64,  133 

Paints,  mixed,  80,  156,  262 
Palladium,  20,  25,  28,  35,  37,  47,  73. 
126,  172 


SUBJECT  INDEX 


341 


Paper,  filter,  adsorption  by,  92,  107, 

108,  178 

opacity  of,  80,  199 
Parkerized  iron,  72 
Particles,  size  of,  Brownian  move- 
ment and,  139 
Passive  iron,  122 
Pectization,  162 

Peptization,  162,  166-171,  212,  221, 
222,  233,  242,  264,  275,  306 
and  adsorption,  166 
by  colloid,  167,  170 
by  ion,  167,  168 
by  liquid,  167,  224,  230 
by  mixed  solvents,  167 
by  non-electrolyte,  167,  168 
by  salt,  167,  170 
Peptizing  agent,  sols  by  addition  of, 

177,  178 
Peroxide,  hydrogen,  catalysis  of,  44, 

125 

Phase,  dispersed,  1,  2,  161 
dispersing,  1,  2 
external,  1,  2,  166,  187,  263 
internal,  1,  2,  166,  263 
Phosphate  baking  powders,  80 
Photochemical   decomposition,    sols 

by,  176 

Photo-electric  fatigue,  23 
Photographic  emulsion,  231 
Picric  acid  and  silk,  105 
Pigment  colors,  196 
Piling,  cubical,  157,  159 
hexagonal,  157,  159 
Platinum,  20,  23,  25,  28,  35,  36,  46- 
51,57-59,94,111,123-126, 
179,  185,  187,  205,  223,  308 
Plasticity,  154-160,  191,  264 

of  clay,  160 
Poisoning  of  adsorbent,  47-53,  5&- 

59,  123-125 
of  haemase,  123,  124 
Pores  in  collodion  membranes,  185, 

240 

Portland  cement  and  gypsum,  80 
Positive  adsorption,  90 
Potash,  recovery  of,  293 


Potassium  soaps,  262 
Powder,  lycopodium,  68,  80,  270 
Powders,  phosphate  baking,  80 
adhesion  of  hot,  81 
coarse  and  fine,  79 
colors  of,  198 
welding  of,  151 
Power,  adsorbing,  of  fuller's  earth, 

121 

Precipitates,  curdy,  162,  163,  231 
flocculent,  162,  163 
gelatinous,  162,  163,  177,  236- 

240 

sandy,  162,  163,  236 
types  of,  161-163 
Precipitation  of  fog,  285 
of  smoke,  290-293 
of  solid,  300-303 
Preferential  oxidation,  46 
Preparation  of  sols,  172-181 
Pressure,  effect  of,  on  adsorption,  3, 

9,  16,  20,  29,  32,  36 
osmotic,    negative    adsorption 

and,  109,  110 
welding  by,  151 
Pressures,   osmotic,    109,    110.    141, 

186,  187,  189 
swelling,  257 
unequal,  154 

vapor,  of  adsorbed  gas,  25 
of  gelatine  jellies,  75,  256 
of  gels,  236,  245-250 
wetting,  77 
Promoters,  56 
Properties,  electrical,  of  sols,  206- 

212 

of  sols,  182-235 
optical,  of  drops,  282,  283 
of  dust,  295-297 
of  sols,   182,   183,   188,   193- 

205 

Protalbinic  acid,  173,  308 
Protecting  colloids,  170-173,  180 
Protoplasm,  308 
Puff-paste,  309 

Purification  of  illuminating  gas,  17 
Purple  of  Cassius,  234,  235 


342 


SUBJECT  INDEX 


Quartz,  25,  72,  97,  179,  302 

blue,  310 
Quicksands,  159 

Radium,  15,  223,  311,  312 
Rain,  1,  283-286 
Rainbow,  197,  282,  298 
Rain  drops,  1,  283-285 

size  of,  284-286 
Rate  of  addition  of  agglomerating 

agent,  220 
Reactants,  41 

Reactions,  metathetical,  sols  by,  175 
Reagent,  Lloyd's,  97 
Recovery  of  potash,  293 
Reduction,  electrolytic,  sols  by,  174 

sols    by,    without    electrolysis, 

173 

Reflection,  selective,  203 
Refractive  index,  188,  199 
Reinsch's  test,  91 
Resonance,  203,  204 
Resultants,  41 
Reversal  of  charge,  219 

of  stabilization,  219 
Reversibility  of  adsorption,  110,  111 

of  agglomeration,  221 
Rhubarb,  295 
Rhythmic  banding.  259 
Ring,  Bishop's,  283 
Rings,  Liesegang's,  258,  259 

Newton's,  197 

Nobili's,  197 

water,  92 

Rolling  drops,  66,  71,  282 
Rouge,  79,  178,  205 

adsorption  of,  by  metals,  79 

color  of,  205 
Rubber,  2,  305,  307 
Ruby  glass,  2,  305-307 

Salt-cake  process,  52 

Salt,  peptization  by,  167,  170 

spray,  281 
Salts,  silver,  85, 98, 107,  111,  165, 168, 

172-174,  176,  177,  183,  215, 

216,  221,  231,  233 


Salts,  thorium,   16,  43,  45,  64,  80, 

114,  169,  226,  227 
Sandy  precipitates,  162,  163,  236 
Saponin,  131,  132,  170,  172,  189,  196, 

269-271,  273,  315 
Savart  plate,  298 
Schoop  process,  303 
Schroeder-Grillo  contact  mass,  49 
Schulze's  law,  112,  209,  213-219 
Sea-water,  iodides  in,  90 
Selective  adsorption  and  catalysis,  41 
reflection,  203 
wetting,  83,  85,  260 
Semipermeable  membranes,  109 
Series,  irregular,  227 
Settling,  consolidation,  193 

subsidence,  193 
Shaking  of  emulsions,  intermittent, 

265 

Shower-proof  fabrics,  71 
Sieves,  liquids  in,  70,  72 
Silica,  18,  20,  40.  160,  177,  185,  187, 
188,  220,  228,  236-238,  240- 
251,  274,  288,  304 
diagram  for,  248 
Silk,  105,  115,  116,  121 

picric  acid  and,  105 
Silver,  62,  166,  173,  198,  199,  203- 
205,    215,    223,    230,    274, 
278,  302,  303,  307 
salts,  85,  98,  107,  111,  165,  168, 
172-174,  176,  177,  183,  215, 
216,  221,  231,  233 
Sintering,  151-153,  200,  230 
Size,  crystal,  and  melting-point,  148 

and  solubility,  148 
of  crystals,  164 

of   particles,    Brownian   move- 
ments and,  139 
of  rain  drops,  284-286 
Sky,  blue,  202,  295,  304 
Sleet,  286 
Smoke,  1,  2,  22,  184,  286-300 

precipitation,  290-293 
Smokes,  analysis  of,  293 
density  of,  300 
wetting  of,  22 


SUBJECT  INDEX 


343 


Snow,  286 

Soaps,  2,  131,  132,  170,  178,  189,  225, 
231,    240,     260,     262-265, 
267-271,  274,  304 
calcium,  262,  263,  267,  268,  274 
magnesium,  262,  268,  274  ' 
potassium,  262 

sodium,  131,  132,  225,  231,  240, 
260,  262,  264,  265,  267,  269 
Sodium  amalgam,  69 

soaps,  131,  132,  225,  231,  240, 

260,  262,  264,  265,  267,  269 

Solid,  adsorption  from  solution  by, 

90-126 
adsorption  of,  by  liquid,  83-89. 

260 

liquid  by,  67-78,  140 
by  solid,  78-81 
films,  89,    189,   190,    195,   260, 

271,  314-316 
precipitation  of,  300-303 
solution,  water  in,  249,  250 
Solidified  alcohol,  262 
Solids,  adsorption  of  gases  by,  3-64 

vapors  by,  3-34 
coalescence  of,  147-154,  161 
gases  in,  304,  305 
in  solids,  305-313  t 

surface  tension  of,  136 
Sols,  162 

by  addition  of  peptizing  agent, 

177,  178 

by  electrical  disintegration,  179 
by   electrochemical   disintegra- 
tion, 180 

by  electrolytic  reduction,  174 
by  hydrolysis,  175 
by    mechanical    disintegration, 

178 

by  metathetical  reactions,   175 
by  oxidation,  174 
by    photochemical    decomposi- 
tion, 176 

by  reduction  without  electroly- 
sis, 173 

by   removal    of   agglomerating 
agent,  177 


Sols,  centrifuging  of,  186 

conductance  of,  210 

density  of,  188 

electrical  properties  of,  206-212 

electromotive    force    measure- 
ments with,  206,  211,  212 

effect  of  gravity  on,  139,  186 

freezing  of,  78 

inhomogeneity  of,  182 

metal  color  of,  203-205,  308 

mutual  action  of,  225-235 

non-aqueous,  230,  274,  275 

optical  properties  of,  182,  183, 
188,  193,  205 

preparation  of,  172-181 

properties  of,  182-235 

stability  of,  212-235 

surface  tension  of,  131 
Solubility  and  crystal  size,  148 
Solutes,  conductance  without,  211 

several,  adsorption  of,  113-120 
Solution,  adsorption  from,  by  liquid, 

126-128 
by  solid,  90-126 

solid,  water  in,  249,  250 
Solutions,  true  and  colloidal,   144, 

183 

Solvent,  adsorption  and,  112,  113 
Solvents,  mixed,  peptization  by,  167 
Specificity  of  adsorption,  111-113 
Sphagnum  moss,  77 
Splashes,  66 

Sponge  structure,  239,  240,  241 
Spray,  salt,  281 
Spreading  of  oil,  82 
Stability  and  temperature,  221,  222 

of  sols,  212-235 
Stabilization  by  colloid,  212,  225 

by  ions,  212-223 

by  water,  224 

of  compounds,  122 

of  foam,  270,  272 

of  jellies,  244 

reversal  of,  219 

Starch,  1,  2,  104,  111,  188,  191,  231, 
233,  240-242,  260 

iodine,  104,  111 


344 


SUBJECT  INDEX 


Static  and  dynamic  surface  tensions, 

131-134 

Stokes'  law,  136,  142,  211 
Strains  with  gelatine,  73 
Strength,  tensile,  of  liquids,  74 
Structural  colors,   196 
Structure,  honeycomb,  163,  239,  264 
of  charcoal,  7 
of  gels,  236-245 
sponge,  239,  240,  241 
Strychnine,  detection  of,  113 
Subsidence  settling,  193 
Sugar,  1,  161,  168,  183,  225,  230 
Sulphate,  calcium,  nitration  of,  159 
Sulphide,  arsenious,  175,  185,   188, 

226-228,  260 

Sulphur,  17,  47,  174,  176,  191,  213- 
215,  223,  237,  238,  274,  287, 
308 
Sulphuric  acid,  22,  49-51,  61,  92- 

96,  115-119,  165,  180 
Sun,  blue,  283 
Sunset  colors,  283 
Supercooled  fog,  281 
Surface  color,  203 
combustion,  62 
concentration,  130-132 
tension,  127,  129-136,  166,  167, 
186,  187,  189,  261,  262,  239 
adsorption  and,  135,  166,  167 
and  discontinuity,  132 
concentration  and,  130,  131 
of  solids,  136 
of  sols,  131 
tensions,    static   and   dynamic, 

131-134 
Swelling  of  jellies,  243,  251-256 

pressures,  257 
Syneresis,  236 

effect  of  ions  on,  244 
Synthetic  charcoal,  5 

Tabaschir,  304 

Tannin,    1,  167,  173,  188,  210,  224, 

232,  260,  266 
Tanning,  electrical,  210 
Tarragon  vinegar,  266 


Temperature  and  stability,  221,  222 
effect  of,  on  adsorption,  3—5,  9, 

13,  29,  32,  113 

Tensile  strength  of  liquids,  74 
Tension,  surface,  127,  129-136,  166, 
167,  186,  187,  261,  262,  269 
adsorption  and,  135,  166,  167 
and  discontinuity,  132 
concentration  and,  130,  131 
of  solids,  136 
of  sols,  131 

Tensions,  surface,  static  and  dyna- 
mic, 131-134 
Tents,  leaking  of,  71 
Theories  of  catalysis,  40-44 
Thickness  of  films,  25,  38,  82,  160, 

189,  191,  314-319 
Thorium  salts,  16,  43,  45,  64,  80,  114, 

169,  226,  227 
Thunderstorms,  285 
Titmice,  202 

Titration  determination  of  adsorp- 
tion, 93-97 
Tree-frog,  201,  307 
Trinitrotoluene,  275 
Triple  point,  adsorption  and  the,  70 
True    and  colloidal  solutions,    144, 

,  183 

Tuberculosis,  294 
Tungsten,  42,  45,  153 
Tyndall  blue,  182 
Tyndallmeter,  287 
Tyndall  phenomenon,  182,  195 
Type  of  emulsion,  determination  of, 

263 
Types  of  emulsions,  261 

of  precipitates,  161-163 

Ultra-filtration,  184,  185,  225,  234, 

240 

order  of,  185 
Ultramarine,  137,  308 
Ultra-microscope,  182,  183,  264,  305 
Ultra-violet  light,  223,  278,  280,  311, 

312 

Unequal  pressures,  154 
Uranium  carbide,  56 


SUBJECT  INDEX 


345 


Vacuum  by  adsorption,  9 

Valence   and   adsorption,  112,   209, 

213-219 
Vapor,  adsorption  of,  by  liquid,  65- 

67,  134,  135 

density,  adsorption  and,  19 
pressures  of  adsorbed  gas,  25 
of  gelatine  jellies,  75,  256 
of  gels,  236,  245-250 
Vapors,    adsorption    of,    by   solids, 

3-64 

Velocities,  migration,  206,  210 
Vermilion,  decomposition  of,  79 
Vinegar,  Tarragon,  266 
Viscosity,  140,  150,  190-193,  268 

and  catalysis,  43 
Vitrification,  137 
Voids,  156-158,  192 
Voltage,  air  films  and,  22,  23 

Washing  of  air,  299 

Water,  color  of,  202,  203 
-in-oil,  241,  261-264 
in  solid  solution,  249,  250 


Water  rings,  92 

wings,  71 

Weights,  molecular,  141,  145,  187 
Welding  by  gravity,  151 

by  pressure,  151 

of  glass.  149,  150 

of  powders,  151 
Wells  near  beaches,  99 
Welsbach  mantles,  64 
Wetting  of  smokes,  22 

of  solids,  23,  67,  69 

pressures,  77 

selective,  83,  85,  260 
Whiskey,  leaching  of,  114 
White  lead,  72,  88,  204,  263 
Whiteness,  198 
Willemite,  15 
Wings,  water,  71 

Woods,  different,  charcoal  from,  6,  8 
Wool,  18,  29,  30,  115-120 

Xenon,  15 

Zero  fluidity,  156,  157,  190,  192,  240 


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